$2500 would be in Box 10, then they show the child care expenses of $2000 on the tax return and the additional $500 that wasnt used for child care, gets added back to income.

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.

Rick