You're real sweet and your friend is amazing as well!
If I were him, would appreciate it if my friends ate at more affordable options, e.g. canteens, hawker centres, and suggest activities that are fun to do but also not $10-20 at one shot. It would help if you act more frugal when you are with him, so he doesn't feel out of place. It is hard to bring up financial matters (especially if you can't gauge what's an appropriate moment to talk about it but HC Tang did a good explanation in his third para) so you can consider writing him an encouragement note and tell him how you are able to support him emotionally/financially.
Honestly, it is hard for a Uni student to suddenly breakthrough financially, but it helps when there are supportive friends.
You're a really good friend and your best friend is very lucky to have you as his best friend. Not becuase you're really well to do but rather he's lucky and blessed to have a best friend that cares and doesn't put $ above friendship / relationship. Genuinne friendship above all else is hard to find :) Cheerish :)
I'd say best help to someone struggling to is to guide / teach him how to manage the sitautions. In this case, could be finance / budgeting.
Ask him out for a drinks or whatever activities / sports events that you guys always hang out where you know he is most relax and happy at, then just casually ask him how is he doing lately, how is things going on, futher probe into the details of things related to the area that you find he need help the most so that you may understand what is the area that he needs help.
if is bill to pay then on the place that you guys go, eg Sports hall / restaurant, then ask the manager or waiter for a help to say and pretend that you guys are the lucky winner today for their anniversary gift and pretend that he has won a cash of certain amount (make sure you've double it so that 50% of the winnings is able to cover the bills that he owe). Next you guys happily split the prize 50/50 such that 50% of the amout still covers his outstanding bills.
For long term financial planning help, you can make excuse and sign up for useful courses on financial /budgeting etc as his birthday or christmas present or call it Buddy's Anniversary gift!