I think your share of 75/25 seems fair, if he is generating the business, and sending you the stuff to code.
your problem here is, if he gets you a client, and you can’t give what the customer wants. for example: a customer wants a fully operational CMS (bespoke) and selling items through Credit Cards.
What if you get a client who want a huge site but only in 5 days?
Will you guys be business partners? if so if the business makes a good profit, and the business gets sold, will you still get 25%?
these are the things you need to consider before jumping into this venture.
but talk to the guy, he seems to want you on board, he may offer better terms