Defining a field as boost removes this field's properties. Thus, it is impossible to sort on the boost field.

boostsorttest.sh

curl -XDELETE 'http://localhost:9200/boosttest'
curl -XPOST 'http://localhost:9200/boosttest' -d '{
	"mappings" : {
		"withoutboost": {
			"properties" : {
				"my_boost":{
					"type":"float"
				}
			}
		},
		"withboost": {
			"_boost" : {
				"name" : "my_boost",
				"null_value" : 1.0
			},
			"properties" : {
				"my_boost":{
					"type":"float"
				}
			}
		}
	}
}'

curl -XGET 'http://localhost:9200/boosttest/withoutboost/_mapping?pretty=true'
curl -XGET 'http://localhost:9200/boosttest/withboost/_mapping?pretty=true'