Yes, do have his teeth looked @. I prefer to have my dentist do mine or at least look at them 2x year and do as little as possible instead of waiting and then doing some drastic changes. While either your dentist/vet is there ask them to age him. Unless you remember him being born his thinning may actually be due to his growing. Is he really 5, he may be much younger or possibly older. Hate to say it but if he is registered and you have his papers do they match him exactly.
Now to feed- All the good quality hay he will consume. If you are feeding hay and he's cleaning it up increase that amount by a flake or more until you reach a point where he almost cleans it all up. Or as long as its good quality give him a round bale. As far as grain a 10% protien level sweet/pellet commercial grain should be fine a high fat level won't hurt. Beet pulp is also very good at adding weight,soak it extremely well for 1hour or more, start with 1 cup dry=3 cups wet that can be increased slowly to 2lbs. Ask about senior and extruded feeds as some of the protien levels are much high which is good for seniors but not good for youngsters it taxes the kidneys to much. Small frequent[4-6] feedings also do much better than 1-2 daily meals as after an hour the feed has past through and they start to feel hungry. Also don't be afraid to work him slowly walk,walk,walk that will help his metabolism and start to increase muscle mass. But do rule out parasites[stool sample] teeth and medical problems. A small vet bill now is much better than feeding for 6 months a not being any better off.