You would still have the run the 401(k) test and see how much of a refund would be due - if it's 7500 or less, then you could do it that way where the "refund" instead gets recharacterized.
An issue could arise if you have multiple HCEs at varying rates/amounts, because of the way the refunds are determined - they come down to a level dollar amount for all HCEs before considering whether anything could then be converted to catchups.
In other words, if he's the only HCE, then 12,500 would work. If he's one of many, then it's not as clear-cut because it's dependent on the other HCEs. If there's another HCE at 200,000 doing something similar then the math does not come out the way you'd want.