I love your avatar picture - what a pretty face!
You seem to be very well organised - well done! It is very difficult, when people first come and post on LB, just how much that person actually knows, so you will have to forgive us if we tell you something that you already have knowledge of. Just remember we all want to help if we can.
Diane has some good advice regarding clipping, if you start with getting him used to the noise and then gently run the clippers over him without actually clipping, just as Diane says, then you should find he will be fine with the proceedure. We clip ours without even tying them up, and they just stand there and go to sleep! You could even cut some of his coat off with special 'rounded edged' scissors (no sharp points), getting rid of the worst of it before trying to clip - will save your clippers getting too hot or blunt trying to get through the thick matts. (but as you mentioned, he may have lost a lot of his coat anyway by now)
You say that you will be putting him out with your other horses - do you have other minis? It is not a good idea to run minis with larger ponies/horses, they may like each other but when having a play, one knock from a big horse's foot can do untold damage to a mini even if the result isn't fatal. Even if you have other minis, then it would still be wise to pasture them alongside each other (safe, strong fencing of course) for a couple of weeks or longer, to give them a chance to get well aquainted before you think about turning them all out together (and to give you time to watch and see who might not be so agreeable to having your new little lad join the rest of the 'herd') Also if your little lad is still an entire, then he should not be mixing with females of ANY age, as not only could he get them in foal, he could get himself badly kicked while pestering them.
By the way - could we know your name?