I have had this situation happen many times all the way back to the Athlon series and in the end , there is nothing you can do. People have a hard time to see value in PC components. For example , "why do I need a expensive power supply?"
I would just tell him , it is your money and get the 9900K
1)seams he does not want to wait anymore
2)ram/ssd /graphics cards prices are acceptable atm , so except inflated intel(production)/nvidia(no competition high end) price , it is not too bad of a time to buy new PC parts.
3)he wants intel and you tried
4)any early bios issues on 3000 series ,you will have to deal with
5)the 9900k is solid , except in value per $.
6)I would only gently point it out if the 3000 series is a lot better, it does not feel good making financial mistakes.
I really do not like it but it takes a lot of time and energy to convince people (including myself) of : "but i used this for the last 10 years".
UPDATE : try what
@CHADBOGA said and then give up if it does not work