i'm sorry to hear that :( granted i do feel marriage should be stronger, and not have someone run at the slightest bump. it'll be hard, but if you two both feel you belong together [and i don't know anyone involved so i don't know how this will end] if i was him, i'd sit his wife and kids down and calmly explain everything. i feel honesty is best, and they all should hear it from him. i'm guessing they're older than i was when my parents split, but they sat us down and told us. i was upset of course, but after a day or two things came around. since there's so many involved, you're going to have to ease into it, and work it out. otherwise, the kids could be forced to choose sides.
is this something that just happened, or a full blown affair? if the second, yea, won't end well. i hated the woman my dad had the affair with [don't have a problem with his new wife though, she wasn't the other woman] still do even after all these years, so i'm not sure how his kids would react. the grandkids, depending on the age, might have some questions about grandma and grandpa, but i don't think they'll be as hurt as his kids.
there's no way to do this without someone getting hurt, and a spell might make things worse. if he really wants to be with you, he should tell his family. also, when he breaks the news, sorry to say, but you probably shouldn't be there, might turn out worse if you were there, since everyone's emotions will be running high. [sorry if i upset you] maybe a little confidence spell to help him a bit, but i wouldn't. it might backfire.
granted i've never been in this situation as the person who's leaving, just as the kid. so i say he should sit everyone in his family down and calmly talk it out. AFTER everything's said and done, and the dust begins to settle, maybe try a honey jar spell to help his family come around if they seem to be choosing sides.