What happened to the other $500? Most employers allow a 2 1/2 month grace period. If it was carried over or forfeited, see the instructions for 2441 box 14.
I've seen a lot of these administrated poorly so if he got the $500 out of the plan without submitting receipts then it would be taxable income. If the $500 came out of the paycheck but was either carried over to 2020 or forfeited you subtract it out in box 14.