Oatmeal is fine, but I've always thought of it as a breakfast food. And maybe for a snack before bedtime if needed.
I always fed my kids the following.
For breakfast: some fruit, and cereal (oatmeal, rice etc)
For lunch: fruit and a vegetable sometimes a little pasta.
For dinner: a vegetable, fruit, meat, pasta or potato (starchy food).
And try some of the foods you eat. Cut it up small. If he can pick it up with 2 fingers he is probably ready to eat it without choking. If not, get a grinder or chopper and make your own foods. I found an inexpensive mini food processor (I think I paid about $4 for it) that works great and is just the right size for a single serving.
I had 5 kids and all of them were very healthy, and enjoyed all their meals. I also babysat and fed all 6 of my grandchildren everyday and fed them the same way. All of them are healthy and enjoy mealtime as well. And not a picky eater in the bunch. All of them love meats, vegetables and fruits.