Of course Yanbu to try to help, but its worth thinking about what that help might look like, and what your boundaries might be?
This isn't meant to sound mean but how were you thinking of helping him? Helping him with what?
It might be that there are some "simple" problems that you can help fix eg books for boredom, posting things, washing things etc. Often having a chat can be helpful, someone I know once told me the kindest thing anyone did for them whilst they were NFA was a specific home cooked meal after they had mentioned it to someone in passing as a food they missed. Would you feel comfortable him using your phone for example? Or giving him a lift somewhere?
I agree the best places to link him in tend to be established services. Your friends could be right in that its possibly naiive to think that you can swoop in and fix homelessness but you might be able to help in some way
Its important though that you ask him what he wants and what might be helpful. Would you like someone deciding whats best for you and that the are going to help you without even asking you for permission or what you might need help with first? Its easy to get carried away with viewing yourself as a saviour, but he is an adult and likely to know best.
People often decide things for homeless people, or think that somehow they will be able to "save them" despite having much less experience of the situation, no idea how service work which can feel very patronising if they have years of experience of navigating systems and are incredibly smart and able,
yet someone else feels that as a passerby that might be able to "fix" it .
For example he might like help filling in forms or equally he might be very educated and find that incredibly patronising. He might want support with housing but might not. Its not a one size fits all
Its not to say you can't help, but tread carefully, respectfully and work together