I wouldn't take John's boasting at all seriously.

He tells Catherine 'of horses he had bought for a trifle and sold for incredible sums; of racing matches in which his judgement and infallibility had foretold the winner...'
Lays it on abit thick doesn't he ? ;)

He probably has a small allowance from Mrs Thorpe.
Many schoolboys overspent their allowance.
In my view, John pretends to be a gentleman by playing cards for stakes, overspending, running up debts to tradesmen -perhaps why he's playing hookey from Oxford ?
Now JulieW has uncovered the implications of the Bedford Coffee House, my impression is John indulges in a fair amount of gambling.
I think that's how he got his curricle hung gig, but I doubt he can afford to maintain it.
I wonder if he can even afford to keep the horse.