If you haven’t fallen out with him already or he with you, it seems that it’s likely to happen.
He may be absolutely straight with this. He may have invested the £500 in the shares but for some reason known only to him, put the shares in his name.
It may be that the price went up from 42p each (1200 shares @ 0,42 = £500 in round figures), to £1.70 and then dropped to 6p.
However, I would want proof of that.
I would want proof of the purchase price and sale price by way of the contract notes. Incidentally, if he was going to manage these for you, you may have a claim against him in negligence and further, it seems that he just decided to sell these of his own volition rather than consult you which in itself may be negligent.
There may be nothing untoward here in which case, he has nothing to worry about.
If he will not let you have this information then you need to make a formal complaint to his principal and if that falls on deaf ears, refer the matter to the Financial Conduct Authority and the Financial Ombudsman Service.
Can I clarify anything for you?
Please don’t forget to rate the service positive. It’s an important part of the process by which experts get paid.
We can still exchange emails if you wish.