It's true - coconut milk is a WONDERFUL substitute to beef up rice milk. (What about oats? oat milk is good too).
Regarding Goat milk, just be cautious as the protein strands seem to be very similar to cow's milk, so may have similar results.
However, I really think your dr should give you strict guidance on this due to his weight and height. I believe there is like an Alimentum Jr. or hypoallergenic Toddler formula that is good for the next couple of years.
Big (((hugs))). I didn't wean off of a formula, but recently weaned of BF'ing and am doing a combination of almond milk and coconut milk. HOWEVER, DD2 is in the 75% for weight, so I was fine experimenting.