It potentially makes a difference in principle whether he gives you 10% of the shares that he already owns or whether you subscribe for new shares in the company which end up with your holding 10%. In the latter case, unless it is part of an approved EMI scheme, then on shares that you receive by reason of your employment you could get taxed as employment earnings on the excess of the market value of the shares over the price that you pay for them. But from the facts stated, unless there is a lot of capital value to the company, it seems likely that the shares are currently worth nothing more than par, which being the case I suspect you are fairly safe.