Oh poor Allen! I hope he doesn't have dental issues already. I have always heard that age doesn't matter with cats. They can have dental problems from very early on. Hopefully he just has some build up and maybe a cleaning will take care of everything.
Don't start blaming yourself either! You haven't done anything wrong. Some cats can live on slop their whole lives and never have a dental problem and yet a well cared for cat can have problems. I think it must be genetic and it's going to happen no matter what you do.
From the weight issue it sure doesn't sound like it's affecting his eating! When he goes in for his yearly just have them take a look and see what they say. I wouldn't worry about it until then.
Good luck on the diet Allen! Just keep reminding meowmie that there is just more of you to love!