My son needs to file his 2017 and 2018 taxes... Is he able to use my program if I've already used it and if so what's the process?

[xmasbaby0]

If you have the desktop software for 2017 and 2018 then he can use your computer to prepare his returns.  A 2017 return cannot be e-filed.  It must be printed, signed and mailed.  

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS received the return.

He can use your 2018 software to prepare a 2018 return.  With desktop software you get up to five federal e-files so if you have not used up your limit of five, he can e-file his 2018 return.  If you have used your limit then he needs to print, sign and mail 2018.