but why have you assumed that the money has come from his "inheritance" just because it came from that account?
he has x amount of money
£3000 came from these savings
£a amount came from his part time job
£b amount comes from you every month (your decision)
£c comes from student loan if he has one
all together adds to his total income.
He could be thinking that he uses the money from you from basics, which he has, and then the rest is his to spend how he likes. In his mind he could have paid for the bag out of his part time earnings, not his inheritance.
Yes its not the best use of money, but I knew lads in uni who could spend £500 on just one or two nights out. A decent pair of trainers could cost nearly £200 quid. At least he's not spending it on coke and porn!
I don't think you would be unreasonable to have a bit of a chat about how quickly money goes, how hard you have to work to earn it, etc.but at the end of the day you have to accept that once he has accrued money in whatever means, be it wages, inheritance, etc. it is his money and he can decide what to do with it. At some point there will be something he wants and he will realise it might have been handy to have that £500, but that's for him to learn.
If you think you are being too generous, cut the amount you give him or, for example, pay his rent up front or load £100 onto a tesco card every month if you want it "your money" to go just towards essentials, but, realistically, ultimately it's all the same "pot" of income.