A magistrates' court is the easiest introduction. The trials are short, perhaps a couple of hours, and are about simple everyday matters of no great gravity .They are more interesting for a first visit to a court.Depending on the court and the time of day, he'll see some pleas of guilty and some sentencing too.(Trials are listed after pleas of guilty, sentences, and other matters.In many courthouses there is more than the one court .In busy courts you may find that one court is doing trials only and other ones doing the rest of the business)What's more, the advocates are nearly always solicitors, not barristers. The ushers will guide you on which court is doing what.
A Crown court trial will run for a day and a half at the shortest. Typically they run for at least two or three full days .So your son won't see a complete trial, unless he thinks of a trial as like a Test Match and can sit, as there, for up to five days for a result, if any! A lot of trials run for a lot more than that. What he may well see though is the whole of a witness' evidence from start to finish with cross examination and all.That can be interesting in itself. The ushers can direct you to what is going on where.(The staff at the Old Bailey have a tradition of telling visitors which scandalous bit of action is going on in which court, and directing them accordingly .Never did all of them do this, just some, but there's no reason to believe that this noble tradition has died out .)
[ Peter Pedant: no judge likes distractions during summing up. At the Old Bailey it has been a standard instruction to the staff that nobody is to be allowed to enter the public galleries during the judge's summing up.That's how important that is.]