Note that the IB carries a fair few credits at Uminn, assuming you have relevant HLS they could be 8 credits apiece
DS will have the IBD plus APs and he really could get done in less than 3 yrs. If you have a score of 39 on the diploma and assuming you have heavy hitters in HL like sciences and math, you might find you have a fair bit of a head start. Uminn cites that you can pretty much shave off a year wit the diploma, but as always, engineering caveats will apply. What does your diploma consist of?