Start with a gift. Continue with some "I thought you would look amazing in this" possibilities from a shopping trip. Slowly work your way into "Can we donate these bellbottoms? You have so many pairs of newer jeans now."
No one changes overnight. If you style your man in baby steps, you may find that after some compliments from people other than you, he becomes a willing participant in the wardrobe change.
He may even, someday, thank you for it.