When to tell your boyfriend?

So me and my boyfrind have been dating for a couple if months but we were really clise friends for two years before any way when i was a child u was molested and its a sore subject for me and the other day we were talking and the question came up about how dar ive gone and i told him making out (which was with him) and he said he has gone all the way and then i quickly got away from rhe subject because it brought up bad memories and i was wonder how and when should i tell him because i trust him completly and i know he respects my boundaries but i feel he has a right to know and why sometimes get "claustrophobic"  around him