So the TC is an interesting one. Not been in front of them myself (I drove PCV and work elsewhere in the industry now) But have had to study them and learn for CPC related reasons. One thing I would say off the beat, is TC driver hearings are generally open to the public, so if you want to go with your Son to watch what goes on, you can do (unless it states it's closed doors on the day) and there's the possibly that he may not be speaking to the actual TC, but a deputy on the day. Deputy's have the same power as the TC itself mind, so that point's rather moot.
My understanding is there are solicitors that deal with the TC as well, but unless you're looking at a revocation of the license or a winnable argument, I wouldn't bother personally.
The TC generally tends to take four courses of action depending on what's going on. No action, A verbal warning, A suspension or a revocation of the license. Generally we could rule out revocation and no action as they are the extreme end of the scale for both ends if you will.
The fact your son was driving and I assume, moving at the time will make this most likely lean towards as Newjudge said, towards the 4 week period. Length of service may come into account too with the TC potentially being more harsh based on less experience. One driver I spoke to who had been in front of the TC for mobile phone use (Looking at a bus stop, doors open handbrake on) was given a ban of 5 days instead of 4 weeks due to 27 years of service with no issues, this particular issue also being a report from the company (Most companies are obliged to inform the TC of these actions as part of operator license related duties)
On a note as well, with how the TC is (to my admittedly only researched and spoken to drivers viewpoint) the fact the phone was in the lap shouldn't reduce the violation. Coach/Bus/Lorry drivers shouldn't have the phone on anything but a cradle in a cab period.
This may be useful for a read to give you a heads up on how things work along with some other info
https://www.gov.uk/government/publications/a-guide-to-driver-conduct-hearings/a-guide-to-driver-conduct-hearingsIF you want to see what kind of examples of punishments that have been meted out, this is on the government website as well, however it does not include why they were at the TC, just the punishment.
https://www.gov.uk/government/publications/regulatory-decisions-made-about-the-conduct-of-professional-driversNot really sure of anything else truth be told, I made an account just to respond to this post