Bike recommendation - young and old

I haven’t seen a recent relevant topic, so thought I’d ask the question fresh:

I’m looking for a new bike for my son (16) and possibly one for myself (48). We are both keen amateur triathletes, although his ability and potential far surpass mine. For the moment, at least, we are both focusing on Sprint distance, which involves a 20km bike leg.

Currently, he is riding a 51cm Cervelo S3 (second-hand), is about 168-170cm and still growing. He’s got to the limit of the Cervelo, even with a longer stem and will need a new bike in the coming months. He doesn’t do standalone bike racing, and other than some TTs during lockdown hasn’t expressed much interest. We often go on 70-100km club rides on the weekend.

His grandmother has been very generous and offered £3000 to get him a new bike, and has his eye on the Giant Propel.

Q1. Is a pure aero bike the way to go or would he be better off with a lighter race bike such as the TCR?
Q2. Assuming he ends up being about 175-6cm fully grown, should we look at a slightly bigger bike and swap out the stem and cranks for something shorter while he grows? I’d really like to not have to buy another bike in a couple of years

As for me, I managed to qualify to represent GB at the Age Group World Triathlon Champs last year, postponed to this year. I’d love to have a great racing bike to use in October. Currently I ride a Canyon Endurace, which is comfy, but not all that fast. Other than racing, I do a mixture of riding, including 160km/100 milers with my club. Is there such thing as a comfy aero bike? Or is my aging back going to regret it in a couple of years?