Did your H specifically indicate that he was wanting to take you out for mother's day? That's not clear as perhaps he wants to get everyone together because of S's b-day and your H is simply wanting it to happen on mother's day.
While that would be something to note, your H could simply be doing "the right thing" in honouring you as the mother of his son. Nothing more, nothing less and it has no bearing on whether your H loves you or not nor that he wants to reconnect with you.
Do what works FOR YOU. If you go, just be sure not to have any expectations around it. Enjoy yourself and just don't expect this one event to change your H's mind about his future.
If you don't think you could handle being with him, that you would be edgy or emotional, then decline. If you can manage it emotionally, it could be a good time.
