Hi Gareth
The physical chassis are the same size (Jnr/Snr) it's only the engines that are different (& some regulations etc).
At 15 I'd say just go senior, no point doing Junior for a year or whatever as he'd do better putting the test time inn and getting up to speed in what he's going to race going forward.
There are 3 main classes in MSUK racing
Senior Rotax (Senior Max) with 2 variants based on weight
Senior X30
Senior TKM (TKM Extreme Maybe)
But there are a few other classes as well including Senior Pro-Kart which uses twin Honda GX engines.
As many will say, go to your local club/track (or the track you think you will start at) and see what classes they do so you know what is available locally.
Costs wise I guess it could be (most expensive to cheapest)
Iame X30
Rotax
TKM
ProKart
I'd suggest to start with dont get all tied up with MSUK racing. Do some local IKR (Independent Kart Racing) at tracks which offer these championships, get used to the kart, learn for a year or so and then you will have more of an idea of the karting scene.
For a test day out it can be cheaper than a day shopping with the family (so my dad says).
Hope this helps a little bit but one final note, there are loads of people who know it all in karting and will sell you a 'great deal'. Remember the motto buyer beware, look around, talk to people but always try and seek good advice from those not trying to earn something out of the deal.
Karting is fantastic, I love it and I'm sure you and your lad will to. Theres so much info on the internet now that you can find out loads, for those questions you cant this is a great forum and then there's guys like Alan Hoyle, Rob Hanscombe and Jack Dex who are in the sport and will give you an honest opinion.
Any questions just ask