So let him hold a grudge, what is the worst that can happen ? It's not like the world will stop spinning because FIL is mad at you guys. This is a part of growing up and being a parent, looking out for your child's best interest.
I know you are looking something to tell him so he is offended the least, however because of his history, he is eventually going to be upset about something. Might as well learn to let it roll of your back now.