So you know how all MN husbands cycle.

In my local area there are several charities that take in and refurbish bikes to sell on and often have some nice road bikes available. It would be worth finding out if there is something similar near you.

As light as possible.

Road tyres.

Will it be just for triathlons or also for other stuff?

lots of gears if planning to do triathlons with hills.

eBay or Marketplace as people are always changing. Needs to be the right size. Beware that the budget goes out of the window if this becomes serious

https://ebay.io/m/vqfFbY

One of many. Disc brakes in theory better than rim. Any triathlon clubs nearby who could advise? £600 gets a potential starter bike but it is an arms race re bikes, wheels etc

You need to figure out the size first - usually done by frame size, e.g. 54cm or 56cm.
Then look for an older bike from a real bike brand, eg Ribble, Trek, Specialized, Bianchi.
Ebay is pretty good if you don't have a local second hand bike shop

If buying second hand do your due diligence to check it isn't obviously stolen: https://mynextbike.co.uk/blogs/articles/avoid-buying-stolen-bikes
https://www.bikeregister.com/

My DH does triathlons and he would advise going to a good bike shop that also does fittings - they charge you for the fitting but take the cost off if you buy a bike from them (so he says) and then you work out your size and what style you like - apparently everyone is different but it’s true that light is what you want and a road bike and maybe aero bars (sp?)- and if it’s too expensive from that shop start looking online for pretty much exactly what the shop had but second hand - he has 2 second hand bikes and his first was around £800 a long time ago and his second when he got even more into it was £1500! Ridiculous money but it keeps him off the streets 😁 then take your second hand bike back to the fitter and they adjust everything until it is just right as when you cycle 100 miles ( or whatever) every little change helps and I think the fitting was under £100 so as long as the second hand bike is cheaper you come out on top iyswim

Decathlon sell second hand reconditioned bikes of all brands - may be worth a look as they’re an established, well known retailer. (They also sponsor their own racing team)

Decathlon! Their road bikes are excellent even at entry level. You get twice the bike for your money as anywhere else. They actually provide bikes to some racing teams and their top end ones are really good

Local cycling club - their members are usually trying to move on a bike to make space for the 'next one'?

1500??? Just about gets a decent set of wheels in the Tri world.

How old is your son? How tall is he? That will make a massive difference in terms of adult v childs frame and therefore cost.

I second seeing if you know anyone in a local cycling club...we have a large youth section and someone is always selling one as their kids grow

Well - that’s what he told me that it cost second hand - should I go home and question him? 😆 I don’t really mind but he did say it was worth $2500 new - maybe he spent more and didn’t want to admit it to me! It’s so long ago that it’s all water under the bridge and perhaps bikes used to be cheaper years ago(?)

they were but 1500 s/h sounds good. Never get back what you paid.

Check if disc brakes are allowed in races before going down that route.

Some good advice here:

https://www.cyclinguk.org/article/cycling-guide/buy-secondhand-bike

Worth asking in his club, which is likely to have an active buy and sell culture. People are always replacing stuff and want to sell on with the least hassle.

Enthusiasts forums usually have selling sections which are lower risk than FB etc. Members will have a keen eye for anything dodgy and hound them off the site. Plus if you're a thief, you'd be exceptionally stupid to try and sell what you nicked to the person you stole it from or one of their mates.

how old is your son?
what is his height.
where in the Uk are you?

getting a road bike is a good step
joining a youth cycling club is also a good idea- giving him opportunity to train on closed circuits, learning race techniques and bike handling.

if you answer the first questions i’m happy to help.

i see that others have asked these questions.

but also that you might have been scared off by the talk of £1500 wheels

it’s absolutely true that you can spend £££ on bikes.

but my kid is currently racing on a £300 13 year old carbon frame, and is making a name for himself.

VC167 are based in the NE. They do longer distance cycling but being able to ride a bike for 50 or 100km is good practice. They'll be able to give some good advice

https://vc167.org.uk/

Second hand islabike (Luath). Have a look on fb marketplace, they hold very good resale value so if he grows out of it then just sell it on for similar price.

Oh, just read your post properly - editing to say they are sometimes sold on ebay too if you want to avoid fb marketplace.