It seems to me that the problem is that he is used to the type of money that can be earned self-employed. Although he would be using your kit so wouldn't have the expense of running your van/making the water for your work etc., the fact is that he is still stuck with the fixed costs of actually BEING self-employed (i.e. the fixed costs for his own vehicle are still the same, his PL insurance is still the same etc.). So really, I suppose the figure that would need to be calculated to find a fair rate for him are to calculate what he normally earns, deduct what he is costing you for using your kit, factor in some profit for yourself, allow for the fact that he will have no paid holiday leave (compare to PAYE on this one).
The hourly rate that would be left is anyone's guess but it should be quite a bit less than what he would normally turn over in an hour but quite a bit more than he would get on PAYE. It sounds highly likely that even allowing for using your kit, the drop would be too much for him and the same rate might too high for you to profit.
I can't go along with the £10 an hour mentioned earlier though. If you had him on PAYE and paid £8 an hour, that extra £2 an hour would barely cover the holiday he doesn't get.
It may be that a number around 16-£18 an hour would be thereabouts but as I say, maybe there isn't enough wriggle room for you both.