@TwoBlueFish
Personal budget is used as an old generic term to call his weekly amount, ie how much the direct payments per week are from ASD. Or how much his weekly funding from CCG (NHS) CHC funding, which is called PHB Personal Health Budget (not direct payments). They are slightly different as funded & commisioned by different agencies (ASD v. Health CCG) .
OP will know if her DS is fully CHC funded by PHB (CHC health) rather than ASD. (Or it could be joint funded which makes it little bit more complex but usually SW is first port of call then).
OP is DS's transport to/from college his 5 days a week funded already without significant cost to DS? That's what I would suggest enquiring about, if it's not. It sounds like it's all new, if he's only just claiming Pip. (Is he in the LA or health transition team from child to young adult? They're used to dealing with all those type issues so do ask them!)
Generally speaking transport to college or day services has a stronger argument as an included additional special transport cost within assessed eligible care support needs.
(Since he is now getting Pip mobility to pay for transport to/from social & generic activities via high mobility rate is £59/week- or you could get IMO great value in using it to get motability car so you only pay petrol costs).
They key factor in your OP is you're really being charged far more than others for mileage. If you employ your own PAs, how did that happen? It's definitely above HMRC mileage rate, and presumably they are also being paid hourly on top for time driving & receiving above HMRC mileage rates. So that's a bit
. Definitely get some advice from your independent living/direct payment support service on those costs.
Also, am I getting confused because in OP it was mentioned 3 long round trips at week (to bowling?), which seems a lot to fit in an already full college week. Or was that just for school holiday period? As additional support costs (activities and transport) when college is not running, is an argument that can be put forward in itself.
(I get the one long trip and some more local ones during term time, not so much the 3 long round trips each week if he can't go out weekday nor Sunday eves).
The more you unpick, set out & can explain the neccessity of it, and calculate it out clearly & reasonably, the conversation you can have with your ChC nurse or Social worker, whomever funds/assesses level of his payments.
IME social workers & nurses do listen very carefully and explore options with you.
Does any of that help? I can only speak generally about how it might get teased out.