Have you tried going to an outdoor shop like Millets? I got a really warm, fleeece lined waterproof coat for ds, very reasonably priced, and actually looks quite cool....if you like the 'just about to climb a mountain look' ...which I do!
I got DS a lovely big, cuddly jacket from Zara for £12 on Saturday. They do it in navy or red (we opted for red) - it's like a classier version of a puffa jacket with grey fleecy lining, very roomy and cosy looking, super warm and lovely. Also got him a lighter jacket - a grey and black checked raincoat - also for £12 from Zara. Bargain.