Hello, I'm Keith and happy to help you with your question.
Yes, it happens all the time, it's one of the realities of life especially when the matter spans tax years. You need a P45 immediately to get your tax right in a new job.
If he doesn't pay you it's very simple, sue him for the outstanding commission in the Small Claims Court. It costs peanuts to do this and you get those costs back as part of the claim anyway. Claims for unpaid wages are invariably incapable of being defended. Make sure you keep all data in respect of outstanding commissions. Be fair though, he may be perfectly honest!