Guys...what can i tell my ex?

after he broke up with me he regretted it, and started IMing me, commenting me, and so on, and talking about sad songs that make him think of us or w/e, and then he stops out of nowhere. his friends tell me he feels guilty for hurting me, but i still dont know the reason as to why hes ignoring my IMs, only the third time i IMed him he replied and only "my sisters on the computer", should i just back off, or show him i care and i wanna know why hes ignoring me? and if so what should i say?