I'm less concerned about this ribeye dinner and way more concerned that you are engaged to, and have bought a house with, someone who is not honest with you about finances and other issues. Surely, in the years you have been together, you have seen him eat. Surely you knew this was a problem. Yet you have not talked to him (and vice versa) about long range goals, values, role models among parents & stepparents, budgets, financial priorities, and the like.
I think discussing weight is only one tiny piece of the puzzle here. It seems that you have gone into the real estate venture and are just starting to see what living with him is like. And you're just finding out that he can't pay some of his bills. Maybe that money is going into excess food, maybe it's going into other things. You have some contradictions in your post: he "takes care of his business" but he can't make his bills, he's "honest" but doesn't disclose things to you that affect your life, he values "open communication" but has been holding back on some facts. Maybe he doesn't want to hurt you, maybe he thinks he can handle things himself, maybe he's just starting to trust the relationship enough to show his full self, "warts & all."
I think you could benefit from couples counseling and from financial planning assistance to set a budget. That budget needs to include everything, from mortgage/utilities to food to vacations to upcoming expenses for your son (camp, orthodontia, college).
This isn't about the birthday dinner. It's about much more. Please get some professional advice to help you two start your married and residential life on a firm foundation.