Both good looking horses! Not obviously unhealthy looking. The mare looks to have a roughish coat, but could be the season or her age.
I presume the horse being ridden is the 'skinny' one? He does look a little light, tho not at all bad, from what can be seen, saddle & rider being in the way. He looks perhaps more a finer, thoroughbred-y type, and also given his age, he could be just having a 'leggy teenage phase'. So if he's got enough roughage, he's wormed & teeth are in order, I'd probably forego the extras, at least for a bit, to see how he goes. As with people, lean is healthier generally, and also trying to 'beef up' immature horses risks developmental orthopaedic disorders(you will find lots of info on 'DOD's').
By light riding, at their age, I mean very
light, such as sitting on them & going for a 5-10 minute walk every now & then. I'd wait another year or 2 for light 'real' riding
& don't mean to pick, just not sure how much you know, whether this is just a 'one off' or some such... It appears that saddle may not fit the horse & it is sitting forward on his shoulder blades, which would be uncomfortable. Also a 2.5yo baby, even if started really early, & 'quiet', hasn't had a lot of experience/education, and if your son's not an experienced rider/trainer, probably best not to let him ride much(especially without a helmet!), as everything you do with a horse is training it, and if the rider doesn't know what he's doing, the horse isn't sure about it, it's a good way of inadvertently 'untraining' a horse to be a good mount.