Op, if he wont listen to you, you need to get him to talk to an advisor or someone else in the family who he may trust.
The money needs to be invested as otherwise he will fritter it and regret it bitterly when he wants to buy a house. Also, as someone else said, tell him to not discuss it with anyone.
Regarding the historic investment, trustees have a legal duty to invest money as best they can, so if they have been managing it for 9 years, and there is no growth, they haven't done their job correctly. Did they also pass him statements of where the money had been, if not you can ask for them. Again, they should have kept these as part of their trustee duty.
My 2DC got an inheritance 2+ yrs ago. DS had just turned 18, and with our advise he is drip feeding it into a stocks and shares ISA and the rest is split into savings accounts of 1-3 yrs.
DD is just approaching 18, so her money was initially under trust of solicitors, but they transferred it into trust of DH and I almost 2 yrs ago. We have it split across some 'child' accounts and again drip fed into ISA.
DS is just finishing second year at uni, and it hasn't affected his loans, as was your original question, only our income has done that. TBH, apart from making sure his ISA allowance is moved into his bank account each month, he generally ignores the inheritance.
Sadly, my niece who also inherited, died recently, unexpectedly. My DB is trying to sort out what accounts she had, but from initial checks, it looks like she kept majority in main bank account and frittered a lot of it, on holidays, meals etc.
However, her sudden death at a young age has also highlighted to us the need to have some life, and not necessarily save every penny, hence my DD has added to what we gave her for a car, to get a newer bigger car. She is having a gap year and will work so will replace the money with earnings. Its a balance at the end of the day. Not frittering it, and being aware of the need for house deposits, whilst still enjoying life when young.