Form 1099 NEC

I see a mix of terminology for Worker Misclassification.

"I am trying to determine what amount if any, the employer reported."

An Employer reports W2. It's possible this person should have been an employee, of course.

"Can the IRS provide me his wage information"

It won't be wages. You described that, already.

"based on what has been reported under his social security number?"

Your taxpayer needs to get their Tax Transcript to see what has been reported for them for 2020.

Can the taxpayer check his bank account statements?

Taxpayer is at fault here, too for not keeping records. His income is reportable whether or not the payor issued a 1099.

Are you on bad terms with the old boss, also?  You can jump through a lot of hoops suggested here, and maybe not find out anything even by July, or you can write a short letter asking the boss if a 1099 was issued, and if so could he please send another copy, and if not, could he give you a figure on how much your client was paid.  Probably a good idea to have your client sign a statement beneath your signature saying he gives you permission to receive this information.  Enclose a stamped, self-addressed envelope.  

If you end up having to file with an estimate anyway, it would show that at least you tried.