He especially bites men and will bite any chance he gets. If you just dab at the horse to tell him to quit, it will just invite him to play more. Horses bite each other in play. It is best to discourage this when the foal is very young and never let it form into a habit. BUT young horses are easier to train NOT to bite than older horses.
Stallions tend to bite more often than geldings.
Also, please let me know your level of experience training horses. If you are not a very experienced trainer, this might be a problem that you will need a professional to help you with. If the horse is getting worse, this is not good. Also, I am puzzled why you don't want to put your 2 horses together unless the older horse is big and mean and the younger horse is just a weanling or yearling. You know, sometimes, older horses train the younger horses to have good manners.