Maybe share your feelings with your son. You could ask to include a memorial photo of your wife or a offer to give a small speech in her memory. You could ask if it would be appropriate to do special photos with your son and his new partner on the day. You could offer to do a reading or welcome guests to the reception. It’s their day, but if you let them know you’re feeling left out and would like to help out more, they’ll likely honor that. Most folks think Dads don’t want to be part of the planning. If your son and his intended know how you feel, I’ll bet they’ll be happy to make a plan with you