I have joined here because I keep reading things about myself (thank you) and can't resist joining. For total clarity, in case anyone does not realise, I am a retailer (have been for 14 years) and was a teacher for many years, first instrumental teaching, mainly violin, then after that school music teaching to A level (also I was a viola player in three different symphony orchestras of the good amateur type - these days I am riddled with arthritis and can't play more than open strings).
I am never going to make any value judgments here about any brand or shop including my own, any comments on these matters will be factual only. Anything else would be grossly unfair and would rightly draw down the wrath of the moderators.
Anyway with that said, here is my comment on the viola size: your son is 5 foot 1 in old money (sorry I still can't think in metres). 15 inch is the absolute maximum size you should be looking at and personally I would recommend a 14 inch (which is the same size as a full size violin - man adult lady players of your son's height would be playing a 7/8 rather than a 4/4 but that isn't an option for viola).
For a beginner. you don't need to spend too much. If you were talking about violins I would warn you away from the £25 ebay outfits but they don't seem to exist for viola so you probably won't go far wrong whatever you get.
Hope this helps.
Liz