I am so sorry for all that has happened. Please do not let your grieving for the mare get in the way of procuring justice- think how you would feel if you heard it had happened again and you had not tried to stop it!! There is no way he would have been allowed to do this legally, I have heard of Dentists giving ACE or other legal tranqs, (Mine will not) but NOT injected drugs- that is a Vets work. This is the sort of thing that gives reputable Dentists nightmares, and you need to do something to help these wonderful people defend themselves. Contact the Association- it will have a website I am sure, do it NOW and put the wheels in motion, it may well be that they will take care of everything for you, right down to compensation, so it will be worth your while.
As to the poor little orphan- ten weeks is not old enough to go without the comfort of milk- my ten weekers are coming in on their own and eating a good hard feed every day, but if they were orphaned I would still have to supplement for a few weeks. I do not suggest you use a bottle, a bucket or a bowl are best at this age. Get him onto a good starter feed, ad lib, as well. He also needs company desperately, do you have someone to go, at least, in the stall next door?? ASAP he needs to go back out with other horses- be wary of the broodmares until he has stopped trying to nurse, if he tries to nurses off the wrong mare he will get hurt!! The very best of luck to you, keep us posted, please.