I'm sorry to hear this and really don't understand why the school is being so difficult. Most clubs do day release at this age do yes unfortunately it would be the same thing. Apologies OP but I don't know any schools that haven't released. It's a shame the trial didn't happen in the summer as training is in the day - but I think you mentioned your son was injured....
Maybe negotiate the trial time and then agree to meet with them once a decision is made? They tell you absolutely nothing through the trial so it's hard to get an idea, but you should get a feeling. As mentioned, we were lucky as my son knew after about 3 weeks so by the time he started to 'guess' the decision was made for him.
Otherwise, and it isn't ideal but can your son move schools? You could 'interview' another school with this criteria in mind? I know this is getting ahead of things but you will need the school on side as there will also be mid week games that your son will need to leave early for and nationals which will require time off etc.
It would also help to know how he does academically as this can make a difference.
Have a meeting with them asap and talk this through. Consider your options and that you may have to tell them that you may consider walking away. Also, I know private schools can require Saturday attendance for sport etc. If your son is U15's he is fine as they will play on Sundays, but if he is U16's they follow the 18's schedule and play on a Saturday so it might be good to know this before.
Very best of luck to you and I truly hope everything works out.