If you are going to quit nursing soon, definitely find some good raw milk...as much as he will drink. Do you have goat's milk available? That would be the best option.
I would also be careful about feeding grains. Grains are very hard to digest--for all of us, really--but especially for young children. If you soak them in an acidic medium (yogurt, whey, buttermilk, lemon juice, etc.) for 12-24 hours first, they are digestible. Or sprout. Read "Nourishing Traditions" for more info on that, or see the threads about that on this site. I personally omit them altogether for my little ones, as we have had many problems.
My story: our little girl was born a healthy weight, about 8 lbs, but was very slow to gain. She slept through the night for 7-9 hours at starting at 3 weeks old, so initially I thought it was that. She is also #7 for us, so I also thought maybe my milk was not as rich. But I was so busy, and since she seemed healthy in other ways, I didn't really worry. Also, I realized later, that because I was so busy, I hadn't noticed that she was nursing so quickly, maybe 5-10 minutes at a time. She probably wasn't getting the hind milk that has been previously discussed here, and also, I probably needed to use measures to make my milk richer. Well, at nine months, she seemed alarmingly small to me, so I weighed her. (We don't do well-baby checks). I was shocked to discover that she hadn't gained an ounce in two months! She was only 16 lbs, and very skinny. She was on a few solids, since around 8 months old, but her digestion started getting worse and worse. Her poops were extremely nasty and the foods looked almost like it did going in! (BTW, any time a baby/child chronically does not have formed BMs and/or a very acidic, nasty smell, you can pretty much guarantee a digestive problem--usually the child cannot handle grains). She was only eating fruits and veggies at this time, so I couldn't understand it. Well to get to the crux...when we consulted with a natural therapist (not quite an N.D.), she helped us determine that it was a thyroid issue. I can't even remember all the ways that thyroid and digestion and growth go hand in hand, but this made a ton of sense to us. A simple way you can check anyone to see if the thyroid is funtioning properly, is to paint an iodine patch on your skin. Just buy the regular liquid iodine from the drug store, and use the stick to paint a 2x2 patch on your belly, arm, or some similar place early in the morning. Ideally, the patch should stay on the skin for 24 hours before fading completely away. If it only stays 12 hours or less, you most likely have a problem. Our daughter's only stayed two hours at first!
I can't remember all that we did to remedy it, but these are the most important things. First, we were told to take her off all solids until she was a year. Most likely my milk supply was lacking, so we were told to supplement with raw goat's milk diluted 50/50 with water. Gave her as much breast milk and goat's milk as she wanted. Also gave her food grade liquid iodine drops. NOT THE IODINE SOLUTION FROM THE DRUG STORE...that is poisonous! Simply using kelp is not enough for someone with a true thyroid problem.
I wanted to share all of this because I have learned that thyroid issues are EXTREMELY common, and most times are misdiagnosed. Many children can be "on the small side" and seem healthy, but problems can crop up later, particulary developmetal and learning disorders. [That reminds me...our daughter did not crawl until almost 11 months, walked at 17, and did not have a tooth until over 13 months...and still seemed "healthy". But I believe these things were delayed due to thyroid.] Of course I'm not saying that all small children have something wrong with them, but just wanted to help people realize that it isn't always genetics. They can still be small in stature, but at the same time still having baby fat. That's always a good thing, when it results from healthy foods!
I would continue the good fats, lots of fruit and veggies--with more emphasis on the veggies than fruit--raw milk (!!) yogurt (naturally sweetened or plain) and plenty of good protein foods. Quality eggs are an excellent choice, good meat, and almond butter, if prepared at home with soaked nuts. Don't ever let a child fill up on empty-calorie foods. Always give nutrient-dense ones. I give my older children homemade sourdough bread, but will wait yet with our littlest.
Maybe this won't help Meagan, but maybe for someone else who has similar issues?