Willy's response to Happy's offer is as follows:
You'll retire me for life on seventy goddam dollars a week? And your women and your car and your apartment, and you'll retire me for life!
Willy here draws scathing attention to the fact that Happy doesn't earn very much, is livng on rent, and also has other expenditures that eat into his income. Therefore he cannot realistically afford to retire his father for life - unless he starts earning a whole lot more and cuts down on his extravagant and womanising lifestyle. That is why Willy is so scornful of his offer.
Happy is shown to be a big talker, but he doesn't seem to have achieved very much in his life. He is like his father in that respect. He also seems to share Willy's unrealistic ideas about achieving success in the world, when he declares at Willy's funeral that he's going to make Willy's dream come true. Like his offer to retire Willy for life, this just comes across as another empty promise.