All of mine have began losing baby teeth at 2 1/2 yrs. I was horrified the first time because I thought the colts had broken each others teeth! My avatar gelding will be 5 Halloween, and he needed nothing done yet. My mare is 7 and has not needed anything yet, but I have had many minis who needed a lot of work and had them kept up every 6 months.
Just follow directions on the feed bag for his weight, there are many formulas with measurements to get a good estimate of his weight. Make sure he is properly wormed, ask your vet if your not sure, and hay will give the little ones bellies, but they look round with their winter fuzzies anyway. Just feel ribs, hip bones, withers, shoulders to make sure he is getting all he needs, and it is very easy to underestimate what they need. Probiotics help them especially after worming. Everyone has their own formula of hay, beet pulp, and feeds they give but just keep with what works best for you.