Morning. Glad to have you onboard.
Is your son more interested in acoustic or electric music?
At that end of the market I would certainly advise to buy second hand rather than new unless you can up your budget to around £150. There are some good options at that price. Personally I would look for Squier, Yamaha and Epiphone, they are good quality and established brands which will be better for resale if he doesn't stick with it, or wants to upgrade. I would also suggest sticking with the most popular styles - Stratocaster, Telecaster, Les Paul - they cover the most bases, are easier to find used and easier to sell in the future.
New options: https://www.gak.co.uk/en/squier-sonic-stratocaster-2-colour-sunburst/961844?gclsrc=aw.ds&gad_source=1&gclid=CjwKCAiAudG5BhAREiwAWMlSjH9tlImFAnEEmBeeGacT8AH_L-O9bYEWnopbspNCeLk7A4j0I9H2zhoCM2UQAvD_BwE
https://www.scan.co.uk/products/open-box-squier-bullet-stratocaster-brown-sunburst-sss-non-locking-tremolo-laurel-fingerboard?gad_source=1&gclid=CjwKCAiAudG5BhAREiwAWMlSjF098_hyRhgw_XMEn5gicGoyXZyYQILujD6LN3RKme8NCzqlF3L55xoCW6YQAvD_BwE
https://www.thomann.co.uk/epiphone_les_paul_special_ve_hcsb.htm?glp=1&gad_source=1&gclid=CjwKCAiAudG5BhAREiwAWMlSjJxYi77dA8vWfgQT3ulxvM_UQN0JtUM1hreI7L3D4Z07bWqrn03cjBoCyncQAvD_BwE
https://www.mannsmusic.co.uk/guitars/electric-guitars/yamaha-pacifica-012-red-metallic-electric-guitar.htm?opt=15172&gad_source=1&gclid=CjwKCAiAudG5BhAREiwAWMlSjPOMok3yGwyks78Iu27G4jK3QDTmB8tmQV1jzNl9e9C6ggCPLxCLfxoCtoQQAvD_BwE
As regards acoustic options, there is more choice at that price range. I would look for Fender, Yamaha and Tanglewood. Again, second hand would be a good starting place.
https://www.fairdealmusic.co.uk/products/fender-cd-60-dreadnought-acoustic-guitar-cherry?variant=44092501491937&country=GB¤cy=GBP&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&gad_source=1&gclid=CjwKCAiAudG5BhAREiwAWMlSjMc6GeJQ4Aq6_EAhRUCY5-_5ovUJm4UWp4J3ItKzxLo9vnwN7Yb-QBoCSJQQAvD_BwE
https://www.amazon.co.uk/Yamaha-F310-Acoustic-Traditional-Western/dp/B000RVYW7E/ref=asc_df_B000RVYW7E?mcid=f713fbf20e48303e87c76ca5b892c1c5&th=1&psc=1&tag=googshopuk-21&linkCode=df0&hvadid=697322390111&hvpos=&hvnetw=g&hvrand=9317386785188518165&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9045298&hvtargid=pla-309497947402&psc=1&gad_source=1
https://www.gear4music.com/Guitar-and-Bass/Tanglewood-TWCR-O-Crossroads-Orchestra-Acoustic-Whisky-Burst/1DQY?origin=product-ads&gad_source=1&gclid=CjwKCAiAudG5BhAREiwAWMlSjHvLBWF_v_nl6HQJMNHl3ekFxkkSEPt8yJgUcVWIakVippJs9ek0hRoCzUkQAvD_BwE
There are other brands and options available and others might suggest other options.
Depending where you are in the country, you could actually go into a store for your son to play a couple and see what fits. Difficult for a beginner to make a choice, but any of those instruments should be ok to start the journey on and 100 times better than the instruments we used back in the 80s.