I’m going on a few assumptions, based on my observations of youngsters around his age. I am not a youth coach, never had been, and have no experience coaching youth.
However, there are commonalities that I have seen that warrant some suggestions in this regard of your question(s) and I would like to take parts of your last response to base my observations on.
This is an awkward age for most boys.
Muscular coordination can be a balancing act at times. Couple this with pitching off a mound can be a test of this coordination and balance. I say this because – have you ever watched a person hurry down a flight of stairs without holding on to the banister? The first thing they do is put out their hands, usually down with the palm of their hands facing the ground. This is the body’s natural response to defend itself from falling. Mature pitchers who have sustained leg injuries with either their knees, ankles, groin and such, natural swing their glove arm/hand out and down as an involuntary response to feeling their legs not being able to support their stress-loads.
But when throws from a full motion he seems to revert to more of a windmill.
The human body/mind has a tendency to be a product of repetitive learning, while under stress. Stress forces us to revert to what we know best, either as a defensive move or what has been used in the past. Your youngster may deal with stress in this way by doing what he knows, simply as his response of “dealing with it.” Now, he’s probably not in a pressure cooker situation, but to him nerves may play a role(s) in his demeanor, thus wind milling. as you put it.
A bit on the husky/stout side, but not overweight.
Husky/stout physiques are usually less flexible than most physical makeups. Couple this with his age -14, he is probably dealing with as much as he can, the best he can. A youngster with girth is very difficult to coach, pitching wise. Now from a football standpoint, being all that you described, I bet he is not all that bad as a pull guard third and long. (or something like that.)
Finally, I wouldn’t be surprised if pitching wasn’t his first choice as a ballplayer. I say this because usually youngsters that “big” for their age at 12-14, usually can command some good heat, when compared to others. Maybe not the best heat around, but given the overall population available, these people are assumed to be one of the options. I would ask this youngster if he really wants to pitch, or would he prefer to be a position player.
By the way, how is he at bat? Does he display some of the same tendencies from a batter’s perspective?