If he wants to buy something and needs a loan, and you can afford it, this is what I have done:
22 yo grandson needed a hefty deposit for a flat. He has a great tech job with a good company and fine prospects, but is still not financially secure. I loaned him the deposit and he set up a Standing Order to repay me.
20 yo granddaughter needed a car to get her to work, after the horrible shed she had, blew up. I loaned her the money under the same circumstances. In both cases, I make no charge for interest, I just want them to get a good start in life and not pay exorbitant interest fees to either a CC company or a bank. Both of them are sensible young people and both have good jobs.

My son is currently struggling with an old car and I know that I may have to repeat the procedure with him, probably next year. We are a close family who help each other. Please tell us OP, how old is your grandson? To demonstrate the effect of opening a young person's mail, this is my experience at 15:
My domineering mother took it upon herself to open a letter from my first GF and burn it. She took pleasure in telling me this when I came home from school, claiming that what was written, was "Filthy and dirty" (her words) At 71, I can still recall the effect upon me at that age. I went to sea at 16 and joined the Army at 19. My resentment against this and other events, lasted years.

Probably not the same circumstances, but mother was trying to "protect" me. Ask the lad in future, if you can read his mail, but first take the advice already offered and tell him why you did it and what you are trying to do for him, then teach him about the traps that young people can fall into. I know you mean well, but how do you know how he secretly views your actions?