I would tell him to do what makes him happy.
But if he's playing every week in the u/23's yes he will get game time, but it's still not comparable to playing against men, and having the fact of it actually meaning something.
It's a gamble....and I hope it happens to my lad so that he has that choice. If it came along, I think my son would take the money and prestige of the premier league side. It's going to be a very confident individual that believes in himself enough to think he will be good enough to get game time and earn his big pay day the hard way.
5k a week over three years is £750k! He'd come out with £300k in his hand....however unless he starts knocking on the door of the premier league side by the end of the three years he may find himself dropping down the leagues and gone. Whereas if he gets experience and games at first team level he may stand a better chance of making a full career out of the game. And even down in league one mid table sides, these fellas are earning £200k a year.
You get my drift....who knows what's the right thing to do, nobody has a crystal ball after all. But I would never criticise any lad for taking the cash....you're a long time out of football and the rewards are huge and can set you up for life.