Most you can do is;
Change email from his to yours, then to another email.
Change password twice, first one a temp, then 2nd your real one.
Set JAG.
Buy some sort of product with a card or SMS, so Jagex has your payment information on file, and can help secure it, usually people pay membership with pins, so if they never had their info on it, they may ask what info you used if he tries to recover. Also, login, then out a few times, to sort of lock in your IP if that makes any sense, other than that.. Idk? It may or may not be safe, just be careful, if you ever need a MM click middleman in my sig and I'd be glad to help.