This comment isn't intended to be a criticism of you - I know you have done your best. This comment is intended to be purely about the future and where you go from here. It is meant kindly.
You are creating a financial mess for yourself and your son. His PIP is money for his needs which you are his appointee for (I assume). You are spending some of your own money on his needs so then saying we'll it's not simply his money ....
You need to separate the two things. Imagine you were told today that in 12 months your son's finances and your behaviour as appointee are being audited. What would you do differently?
Whether that money hits an account in your name or your son's (which I think you would need power of attorney from the court of protection to manage so is perhaps not feasible at this time) it needs to be kept separate and managed appropriately.
You need to be confident that if the authorities ask you "can you show me how you, as your son's appointee, have managed his pip. You can reply "Yes - here is the account his pip goes into. Here is what he has spent it on (records/book/budgeting app).
Under your current system if a day service/ social worker/anonymous tip off accuses you of financial exploitation and you are asked to demonstrate how your son's PIP is being used what are you going yo be able to show them to demonstrate that your son is not being exploited?
You must also remember that you are managing the money on behalf of your son - and he should be included/his wishes paramount in any decisions. You say he might work so he has a good degree of capacity. You should help him manage his money- not take over. So if he wishes to buy a holiday or a computer and you would prefer him to save his money you need to discuss it and try to help him see the consequences of his decision but an adult has the right to make a bad or risky decision.