I Honestly understand your boyfriends concernes and inhibitions, we (males) are raised in a society where saying's like -- you can't rape the willing-- etc... are common place and there are attitudes that would keep most men from feeling free enough to speak of thier trauma's. Most men are told over and over again that we should feel lucky to have been introduced to sexuality at a young age. Plainly and Factually when an adult is sexual with a child it is a Trauma for the unfortunate child regardless of the child's gender. The occurence of incest by his grandmother makes matters much worse for him. One book that i highly recommend would be * Victims-No-Longer* By Mike Lew. This is a book for men dealing with issues of sexual abuse and Incest. I found it helpfull to me.
On the topic of sexual stimulation or unwanted erections, physical responces etc.... He Was A Child, Yes children can be stimuated and the erection that occured is completely normal. The erection would happen whether or not he wanted it too. In Other words just because he had an erection does NOT mean he enjoyed it. His body did as it was designed to. Sexual stimulation causes the erection -- not his mind. Shouled he wish to discuss Body Betrayal , PLEASE let him know that I am here and THAT HE IS NOT ALONE.

Good Luck and encourage him to study up on the topic, this may help him dispel some of the shame and self blame associated with having been abused.