Friend of my ex bf sent me a friend request on FB... Should I message him explaining why I can't accept it or leave it?

I haven’t spoke to my ex or his friends for months now. I deleted them all off Facebook a while ago and one of his friends has recently sent me a friend request.

I’m wondering if I should message him or if I should just leave it. It’s been bothering me for a while now and I feel a bit guilty ignoring him because he was quite nice and supportive when me and the ex broke up. Should I explain that I’m sorry about not accepting his friend request but I prefer to keep the past in the past. Wish him the best etc. Or should I just not bother and just ignore him? What is the best and most decent way to go about this?