Pretty sure we can't recommend other retailers/builders as per forum rules though if you're looking for something to stand the test of time and is somewhat budget friendly I'd recommend speccing out one via overclockers or elsewhere using the custom part-chooser tools that many offer - Ideally you'd like to go for the intel i7 series chips, either the skylake or kaby lake series (6700/7700) both in the region of £200-250 with a respectable ATX motherboard (for the skylake series you can use either Z170-boards or Z270 boards) Ideally an ATX version - Kabylake does perform better on Z270 boards and will also not be so much of a bother to setup or incur extra setup-costs. you could step this down to the i5 6500/i5 7500 if you can't afford the i7 series, all of these will require an aftermarket CPU cooler as they do not come with a stock one due to the thermal/cooling demands. (this will be an option via the shop you choose to build with)
Another good option would be an AMD Ryzen 5 1500X with appropriate AM4 ATX motherboard (AMD is usually a little bit cheaper in terms of price) this CPU also comes with the stock cooler, perfectly adequate for everyday use providing it is not overclocked and in a PC case with good airflow.
RAM wise anything in the region of 8Gb DDR4 2133Mhz or more is fine for a starting block, 16Gb is recommended however for demanding games, typically speaking either 2x4Gb kits, meaning two sticks of 4Gb RAM or for 16Gb two sticks of 8Gb RAM - this way it will run in what's called dual channel mode (working in sync with each other) and makes a small but noticeable difference in system speed when loading games/programs.
Graphics card wise + budget friendly you'd be looking at either the nvidia GTX 1060 6Gb model or (as of right now) either the AMD RX 570 4Gb model or the RX 580 Model (budget permitting) - there is the newer variants launched by AMD but those are selling like hot-cakes right now and it is probably best just to wait them out until mid-2018 as more efficient models usually release later.
You'd be needing some sort of SSD windows boot drive and/or M.2 variant (M.2 drives are much quicker than an SSD but also much pricier, they attach straight to the motherboard as opposed to via cables, they do however require a motherboard with an M.2 port and is somewhat more difficult to setup) + you'll need a standard mechanical drive of atleast 1Tb for games.
Don't worry so much about DVD drives, that standard is dying out, opt instead for a good sized ATX case with good airflow + fans, ideally a few 120mm or 140mm fans in the front for intake, one at the back for exhaust and if possible one/two at the top too - this will balance out airflow and make it less of a dust magnet.
Power supply wise, ideally 650W or 750W and check it is 80+ bronze efficiency at the very very minimum. dont cheap out on that too much as it's just not worth it, a cheap PSU can cause all sorts of nasty stuff to happen.
As for monitors, there's a good few deals for some right now ranging from 24" to 27" - if possible grab a 144Hz monitor as this will allow for higher in-game frames per second too (144Hz being the limit at which the monitor refreshes/loads frames)
Keyboard/mouse are usually picked up cheap enough in combo packs, I wouldn't worry too much about them right now being that your son is still fairly young it's probably best to just buy a membrane based keyboard and not an expensive mechanical one though this is up to you