Good advice from everyone, including a trainer but, IMO, not necessarily a professional trainer. If you know any fair-minded and knowledgeable horse folks that might come and give you an assist.
If you're a dog breeder, you have training experience - just need to figure out how to transfer that knowledge to a piece of livestock. Yes, the little darling is livestock in spite of what many folks want to think
Sitting out there with a book or video game is a great idea, but depending where you live, the weather may not permit that.
He is five months old and NEEDS his family jewels to quickly go missing before you start having mounting problems with him. Plus all that testosterone may cause added discipline problems.
Is your dog vet a large animal vet as well? If not, please contact your large animal vet, as he/she might also be of some help in finding someone to assist you. If you can't catch the Little Booger, you can't get him de-nutted"
Training a mini isn't any different than training a big horse. Feeding however, is another matter. Minis are very prone to obesity, therefore insulin and founder issues.
Hopefully the "Mini Crew" will come in and offer up some sound feeding advice
Good luck and please feel free to ask questions. No question is dumb when you don't the answer