Look folks it's about intent, NOT whether the person is prohibited under federal law, familial relationships, or sequence of events.
If a person buys a gun with the intent to obtain the gun for someone other than themselves (even if the person is not prohibited under federal law), and it's not actually a gift, then it is in fact a straw purchase. It matters not if they are father and son, and matters not if the son gives the money to dad before or after the purchase at the FFL. If dad buys the gun with the intent to get it for the son, but it's not a gift, then it is a straw purchase.
Likelihood of getting caught? Probably low, but who knows for sure. Likelihood of being prosecuted if caught? Depends on the AUSA in your district. Consequences if you are caught, prosecuted, and convicted? Felony conviction, federal probation or even jail time, and both you and dear old dad become prohibited persons under 18USC922(g)(1), and neither of you will be able to legally possess any firearms again.
That being said, I am not a lawyer, and if you want the real deal consult with an attorney who is familiar with federal and applicable state and local firearm laws.