My husband successfully obtained his first UK visitors visa before we were married. The key things he will need to prove are:
1. That he will return here - evidenced by a good job (provide letter from employer confirming job and that he has x time off for holiday); money in the bank (don't try just putting a lump sum in, they look for movement in and out over time); ownership of house or land or business, or long term rental.
2. It will be helpful if he is going to the UK for a specific event such as a wedding or christmas or family celebration (provide copy of the invitation).
3. The fact he has a US visa will be an advantage especially if he has been there and returned.
4. The fact that you are not married is also an advantage - less chance of him trying to stay there.
5. If he has children they do not help the case that he will return - they know that Dominicans often give their children to other members of their family.
Good luck with your application.
Matilda