I have to admit, it does sound too good to be true. But some schools are just that good. It's understandable to have doubts about PS working given your last experience and some of the maddening stories we hear about on this board. Sounds like there are some pros to this school:
- A principal who actually knows something about gifted ed.
- The principal is not afraid of radical acceleration.
- They're willing to do part time! (I'm drooling here, BTW crazy)
- It's free grin

And a caveat:

A gifted learner who is accelerated still learns differently, so you have to be okay with your DS perhaps spending some time on material below his level. The acceleration will "take the edge off". But you'll have time to make up for any underchallenge at home. I think having that perfect challenge level for the whole year may be a lot to ask for.

Could you have a face-to-face talk to the principal to see if they would be able to do an IEP every year?