We have two very cobby type ponies each 14 odd hands each. We had one since she was three years old. With her we have made sure that she has enough hay during the day to ensure she is nearly always eating. We also then introduced a low starch / low carbohydrate/high fibre feed for native ponies or ponies at rest / light work and look on side of the packet for recommened daily portion. We then gave this with some chaff, about half a scoop in each feed, along with a couple of carrots / apples, watered this down so as not so dry. So the daily portion of feed (in the packet) was divided between two feeds, then half a scoop of chaff plus carrots etc for each feed. This we did to ensure that she got enough to ensure that she grew properly. Now she is nearly seven and only still in light work we now give her 1 scoop of chaff, with a vitamin and mineral supplement, with omega oil, again split into two feeds a day. As well as ad lib hay to eat as well.
Before feeding your pony see if it is overweight or not. If overweight then feed less than recommened portion and add a mineral and vitamin supplement to one of the feeds each day. This is because a recommended portion will contain x amount of vitamins / minerals, so if reduce the portion, then reduce vitamins/minerals, so adding these in a vitamin/mineral supplement is needed.
If underweight then give recommended portion and see how he goes over say two weeks or so.
Also is he rugged at all? How much shelter does he have in the field? I do not put heavy weight rugs on our cobs unless it is perishingly cold. But I do rug up with lighter weight ones, as this does give protection against the wind and rain.
Horses and ponies use the hay to help keep them warm, so do need to have a constant supply for most of the day, they can go a couple of hours, but not much more without eating something.
The best way to see if you horse / pony is ok,, is if he looks fine, bright eyed, good coat, and no problems that you can see.
Hope this helps, but would also look for a good book on horse nutition, but get one you can understand, there are loads that are far too technical, good for some people, but I find them too difficult,
Good luck with your new baby, hope you and he have loads of fun together.