If you’re new to the sport, one thing you’re going to have to decide on beforehand is your role.
For a beginner, I wouldn’t recommend becoming a committed sniper unless you or your son have a lot of time at home and a big garden to practice shooting with your rifles quite often.
I would advise you look into DMRs and electric carbines for starting new to the sport. Try to avoid tacky plastic guns often found in the pre £100 price range
I’d also advise you to not go for the cool blackout loadout often used by SWAT and SAS. I mainly play games in the night, which is why I tend to dabble in that field, but beginners would want to go for something between MTP and DPM (different types of camo-MTP is lighter coloured and DPM is darker and more forest-like)