Almost certainly yes. If he is operating under some form of contract, express or implied, oral or in writing, to provide you with work or services, and you reward this with some sort of 'consideration' (usually money), then unless you can prove otherwise he will be a 'worker', and all workers are entitled to at least the minimum wage.
Your best chance of proving otherwise lies in abandoning any mutuality of obligation - that is, make sure it is clear that you are not obliged to provide him with work and he is not obliged to do it if you do. But even if both of you could live with this, you could have a hard time from HM Revenue and Customs (the policing body), not to mention the employment tribunal or county court and the various disability organisations, if it gets investigated. It's a pity, because the fact that he has work and is paid for it may be much more important to him than how much he gets paid for it.
You can pay him for his expenses only, without creating a worker's contract, and you might be able to pay him a daily allowance for coming in, providing you don't oblige him to work when he comes in, or penalise him for failing to do so.
Be aware that if you take someone on and then dismiss them because you find you have to pay them the minimum wage, you subject them to an illegal 'detriment', and they may be able to sue you for unfair dismissal. So take professional advice if you find yourself in this position.
