Even with regenerate, what exactly are you regenerating? If the necessary neural pathways for the legs to work never developed in the first place, they couldn’t be “regenerated”. If this was your goal I think you might need to true polymorph a guy into “the same guy but his legs work”
You couldn’t “restore” something to a state it’s never been in. Cutting off the legs and then casting regenerate would “regenerate” the lost pair of paralyzed legs.