SELECT Name, CustomItem__c, (SELECT OldValue, NewValue FROM Histories) FROM CustomObject__c where id = 'xxxxxxxxxxxxxx'
参考:履歴オブジェクトでの関係クエリの使用
https://developer.salesforce.com/docs/atlas.en-us.soql_sosl.meta/soql_sosl/sforce_api_calls_soql_relationships_query_hist.htm