Are you claiming benefits which you shouldn't be getting if you declared this job?
If yes, your position is a difficult one. If not, then understand the power of your position.
This guy sounds like a user,
Do what you have to do, I wouldent feel guilty. You have explained the situation to him.
I would just mention to your customer that you may not be doing their place fo rmuch longer, see what they say. You may get the oportunity to tell them about the "situation" they may offer you an alternative!!!!
He is using you!!
The trouble is you don't seem to want to acknowledge it.
Once you recognise this you will be in a position to sort it out.
As you have worked for him for 16 months, hmrc will be a bigger problem for him than you, if you explain you thought you were a legitimate employee, but have doubts because of lack of payslips etc.
You have been used for 16 months. Wake up. Is this worth it for 10 hours of cleaner rates per week?
Consider this job lost and don't be afraid to telling clients eventually of your predicament.
My crystal ball tells me he will be peed off if you rock the boat after 16 months of giving in and will pretend you are causing him a problem, because of it.
Get a 'proper' job for a proper company or advertise yourself.
Approach him with something in writing. Email or if by letter, registered post.
Don't rely on verbal exchanges cos things can be denied.
Write to him explaining how long you have worked for him. What the problems are, such as lack of payslips and that you now think he might not be paying your tax and NI. Mention the amounts that are outstanding.
Put in it how you expect to be paid, reminding him of his original agreement.
Sit back and wait for the mock explosion.
You need to be clever about this and think of the long term.
Once he pulls a guilt trip on you, let him know you will contact hmrc to find out if they have received payments in your name as it might affect the benefits you will receive if this job doesn't work out . Act slow but think sharp.
As long as you don't keep up pretensions to yourself that this job might work out, you and he will both realise that you can bring a considerable amount of trouble if you start seeking advice elsewhere.
Finally, this guy sounds stupid cos he thinks you are. Use this to your advantage and he will provide you the rope to hang himself.
Don't threaten but sound unsure of what to do in your letter.
Your goal (or mine if in this situation) should be to get him to acknowledge your letter. Even if it is just an apology.
Once you have this in writing he is as sunk as you want him to be.
If he won't put it in an email or letter then a text is the next best thing.
All i have written above assumes you aren't cheating the benefits people.
Oh and the CAB will give you good confidential advice. Go there 1st cos you sound like you need a confidence boost about your position.
Good luck.