At 16 unless he has got massive natural talent your lad will get dropped very quickly on his 1st try on a crit, there are some very good riders at that age who have been riding circuits such as CSP’s in Nelson since they were 8 or 9, its not just about fitness, alot to do with technique/confidence. My lad got dropped consistently all through his 1st year (at 14) and can still struggle in crits. However through joining a club he has developed a love of cross racing and TTs as well as recreational riding.
If he likes cycling/racing I would recommend joining a club, CSP are supposed to be very good in the 16 to 18 age range. They will have good coaches for technique and fitness and will do more activities than purely crit racing. Above all don’t get dis-heartened after the 1st attempt.
EDIT: Tuesday nights at UCLan as mentioned in Mick_rs post above are very good with a range of abilities taking part