I disagree with the "no Haflinger" opinion. I've had the breed for years and also known quite a few Hafie/Shetland and Hafie/Welsh crosses, and I can remember two of them just right off hand that looked remarkably like this (Hafie/Shetlands). And yes, some (actually, many) Hafies are that short - I just sold a 13 hand mare three days ago. Furthermore, silver dapple expresses quite readily on the Hafie Chestnutt genetics as long as the non-Hafie parent passes on a black gene along with the dilution gene. Also, the pony's head and "less drafty" build suggests a "modern type" Haflinger to me. Someone commented he didn't look drafty enough, but moderns are bred to be far more athletic than the original, drafty type. They do everything from dressage to jumping.
And yes, you should absolutely geld him. Whether "stallion" behavior is learned or not, the perception of the vast majority of potential owners is that stallions are trouble. Unless you know for sure you will never rehome this guy, you should give him the best shot at a good home by gelding him.
Finally, I would reverse the statement that "some Amish trainers are gentle" to "some Amish trainers aren't." Through the years I have owned a number of fine, very well trained horses that were bred and trained by Amish folk. Like any other group, some of 'em know what they're doing, and some of 'em don't. Don't base your choice of trainer on "Amish" or "not Amish," but on your own common sense and the advise of "horse people" around you that you know treat their animals well. Use your eyes and your brain, and ask any potential trainer for a demonstration. If he/she won't give one, walk away. Also ask for - and contact - references. Talk to folks at local stables, in local riding clubs, etc. Finally, ask to see the potential trainer interacting with his/her own horses and pay attention to the animals' demeanors. Are they "beat down," or are they alert but calm? Are they head-shy? Spooky? Overly nervous? And don't just look at one animal. Ask to see several. If the potential trainer is genuinely interested in the quality of his/her work and the welfare of his/her trainees, you should meet no resistance to these requests. If you do, WALK AWAY. It takes far more time, energy, and $ to undo bad training than it does to train right the first time.