You have done well with him if you have done it by yourself.
Have a look on the APDT website. All the trainers on there should use positive methods to help you train your dog. If any mention 'dominance' or 'pack leader' run a mile. Bad classes are worse than none at all. They might not take him in a puppy class, but you can attend training classes at any age.
When we got our first dog, years ago, I remember her breeder telling us not to listen to our vet, as they know nothing about dogs. I can remember thinking she was insane, and that they don't go to veterinary college for nothing. However, as time has gone on, and I have begun to learn more about dogs, I have begun to realise that it is impossible for vets to know it all, and some do have funny ideas... dogs don't tend to eat until they are 'full' they are opportunistic feeders and scavengers by nature, so they will eat what is available usually.
My friend has a very greedy dog. He once broke into the utility room, managed to eat a whole 15KG sack of food, plus the sack, and still wasn't sick... (luckily she works ina vets and he was ok) so you might not make him sick, and personally I don't think it's a good idea anyway. Are you worried about him getting bloat? How many times is he fed a day? you could try increasing his number of meals, but giving the same amount of food, soaking his food prior to giving him it, o rputting a large rock (too big for him to swallow!) in his food bowl so he has to eat around it.
Hope this has helped.