An absence is an absence. An absence because the school called you to come pick him up due to behaviors is a suspension. They can call it whatever they want, but it's a suspension. ISS is also a suspension. It all counts towards the total of 10 days per year.
They're *excusing* the absences, in my humble opinion, so that they don't have to deal with- the behaviors AND they don't have to revise IEP/BIP to address these behaviors.
I'd ask to see the school policy that mandates that any child who does not do homework is to be placed in ISS until it gets done, and that said child will be sent home if he/she continues to refuse to do the homework. If it's not in writing, they're handling difficult child differently than other students. I'm assuming it's not written in his IEP/BIP that he will be sent home if he refuses to do homework.
Once you hit a total of 10 days of his "excused" absences (gag) and ISS combined, I'd call for an IEP mtg to discuss change of placement, since by removing him from the classroom for 10 days they have in essence changed it.