Stored Procedure, to create missing columns default values at target database by comparing both source and target databases.

SYNC_ColumnsDefaultValues.sql

--DROP IF ALREADY EXISTS
IF EXISTS ( SELECT  1
            FROM    SYS.procedures
            WHERE   NAME = 'SYNC_ColumnsDefaultValues' ) 
    DROP PROC SYNC_ColumnsDefaultValues

GO
--CREATE PROCEDURE
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
/*
Stored Procedure, to create missing columns default values at target database by comparing  
both source and target databases. 

EXEC [dbo].[SYNC_ColumnsDefaultValues] 'AdventureWork_StableVersion','AventureWork_Target'

*/
CREATE PROCEDURE [dbo].[SYNC_ColumnsDefaultValues]
    (
      @SyncFromDB VARCHAR(200), --Source database name
      @SyncToDB VARCHAR(200) --Target database name
    )
AS 
    BEGIN
    
        CREATE TABLE #AlterStatements ( TSQLStmt VARCHAR(MAX) )
    
    
        EXEC
            ( 'INSERT INTO #AlterStatements
				SELECT 
					CASE WHEN MM.COLTYPE = ''varchar''
						  or MM.COLTYPE = ''nvarchar''
						  or MM.COLTYPE = ''char''
						  or MM.COLTYPE = ''nchar''
					 THEN ''ALTER TABLE ' + @SlaveDB
					  + '.[dbo].['' + MM.TableName + '']  ALTER COLUMN [''
						  + MM.name + ''] '' + MM.COLTYPE + ''(''
						  + CAST(MM.max_length AS VARCHAR) + '') ''
						  +(SELECT ( case when IS_NULLABLE = ''No'' then '' NOT '' else ''''  end ) + '' NULL '' 
		                  
						  --+ CASE WHEN ISC.COLUMN_DEFAULT IS NOT NULL OR ISC.COLUMN_DEFAULT<>''''
						  --                     THEN '' DEFAULT '' + ISC.COLUMN_DEFAULT
						  --                     ELSE '''' END 
						FROM ' + @MasterDB + '.information_schema.columns ISC
						WHERE ISC.table_name = MM.tablename 
						  AND ISC.COLUMN_NAME=MM.NAME )
		                  
		                  
					 WHEN MM.COLTYPE = ''numeric''
						  or MM.COLTYPE = ''decimal''
					 THEN ''ALTER TABLE ' + @SlaveDB
					  + '.[dbo].['' + MM.TableName + '']  ALTER COLUMN [''
						  + MM.name + ''] '' + MM.COLTYPE + ''(''
						  + CAST(MM.Precision AS VARCHAR) + '', ''
						  + CAST(MM.Scale AS VARCHAR) + '') ''
						  +(SELECT ( case when IS_NULLABLE = ''No'' then '' NOT '' else ''''  end ) + '' NULL '' 
						  --+ CASE WHEN ISC.COLUMN_DEFAULT IS NOT NULL OR ISC.COLUMN_DEFAULT<>''''
							 --  THEN '' DEFAULT '' + ISC.COLUMN_DEFAULT
							 --  ELSE '''' END 
						FROM ' + @MasterDB + '.information_schema.columns ISC
						WHERE ISC.table_name = MM.tablename 
						  AND ISC.COLUMN_NAME=MM.NAME )
		                  
		                  
					 ELSE ''ALTER TABLE ' + @SlaveDB
					  + '.[dbo].['' + MM.TableName + '']  ALTER COLUMN [''
						  + MM.name + ''] '' + MM.COLTYPE 
						  +(SELECT ( CASE WHEN IS_NULLABLE = ''No'' then '' NOT '' else ''''  end ) + '' NULL '' 
				--          + CASE WHEN ISC.COLUMN_DEFAULT IS NOT NULL OR ISC.COLUMN_DEFAULT<>''''
							   --THEN '' DEFAULT '' + ISC.COLUMN_DEFAULT
							   --ELSE '''' END 
						   FROM ' + @MasterDB
					  + '.information_schema.columns ISC
						   WHERE ISC.table_name = MM.tablename 
						   AND ISC.COLUMN_NAME=MM.NAME )
		                  
					                  
								END
					FROM ( SELECT    YY.Name as ColType, XX.*
						   FROM  ( SELECT    AA.name as TableName,BB.*
										  FROM      ' + @MasterDB
					  + '.sys.objects AA,
													'
					  + @MasterDB
					  + '.sys.columns BB
										  WHERE     AA.object_id = BB.object_id
													and AA.type = ''u''
								   ) XX,
										' + @MasterDB + '.sys.types YY
							  WHERE     XX.user_type_id = YY.user_type_id
							) as MM
							left join ' + @MasterDB
					  + '.sys.syscomments mSM on msm.id = mm.default_object_id
					        
							INNER JOIN ( SELECT YY.Name as ColType,
												XX.*
										 FROM   ( SELECT    AA.name as TableName,BB.*
												  FROM      '
					  + @SlaveDB
					  + '.sys.objects AA,
															'
					  + @SlaveDB
					  + '.sys.columns BB
												  WHERE     AA.object_id = BB.object_id
															and AA.type = ''u''
												) XX,
												' + @SlaveDB
					  + '.sys.types YY
										 WHERE  XX.user_type_id = YY.user_type_id
									   ) as SS on MM.tablename = SS.tablename
												  and MM.name = SS.name
					                              
							left join ' + @SlaveDB
					  + '.sys.syscomments sSM on sSM.id = ss.default_object_id    
					WHERE   ( MM.max_length <> SS.max_length
							  or MM.ColType <> SS.Coltype
							  or mm.is_nullable <> ss.is_nullable
							  --or isnull(msm.text, '''') <> isnull(ssm.text, '''')
							)
					ORDER BY mm.TableName'
					)
			
			
			----CURSOR TO EXECUTE/PRINT EACH STATEMENT SEPARATELY-----
        DECLARE MY_CURSOR CURSOR READ_ONLY
            FOR SELECT  TSQLStmt
                FROM    #AlterStatements
			
        OPEN My_Cursor 
        DECLARE  @SQLstmt VARCHAR(MAX)

        Fetch NEXT FROM MY_Cursor INTO @SQLstmt
        While ( @@FETCH_STATUS <> -1 )
            BEGIN

                EXEC ( @SQLstmt
                    )
                -- PRINT ( @SQLstmt) 

                FETCH NEXT FROM MY_CURSOR INTO @SQLstmt
            END
        CLOSE MY_CURSOR
        DEALLOCATE MY_CURSOR
			-----
					
			
        DROP TABLE #AlterStatements 		

    END