300k in costs?! ffs.
I am assuming that the challenge to the will is that it was in some way invalid. Ideally, the executors should take a back seat and let the main beneficiary under the will slug it out with those claiming its invalidity.
The normal rule is that the loser pays the winners costs. However, if your son has acted reasonably, I don't see why he should be ordered to pay the costs personally, and they will likely be ordered out of the estate.
If you can give a few more details, I'll try and assist further.