The worst thing you can do is to ask the grumpy old geriatrics who dominate this forum what is trendy. All you got to do is look at the junk up for sale in the SC to see the tat this lot wear.
The best thing is to make a day of it, take him out and let him chose (restrictions permitting) or let him chose online. That will really please him, a watch is person to him, only he really knows what wants today. Whatever he choses, he will have dumped it by the time he is twenty.