In reply to Tree: My daughter is nearly 3 and we had a good 2 week break in the Lakes. Found a cracking campsite and took her up Cats Bells and Castle Crag, both near Derwent Water - perfect for little feet as there is so much to see and do on the way.
We also have a toddler carrier so carried her up some bigger hills - Skiddaw, Haystacks but as she wasn't moving she did get really cold.
We kitted her out in regatta waterproof with removable fleece and a full on internet purchased snow-suit for the big days out (£35). I wasn't prepared to pay out for hiking boots so a couple of pairs of thick socks and wellies did the trick. Would recommend paying out for waterproof hiking boots if he's walking as small feet are easily damaged by bad footwear. Millets (sorry) was the only place that did semi-decent stuff for a 3-4 year old.
Top tips - go really slow, allow lots of breaks, remember for them it's not about getting to the top of a windy, wet summit - it's about enjoying the journey. Remember kiddies have an attention span of one minute per year of their life so it could be boring for him.
Involve him as much as possible with spotting nice rocks, animals, get him his own camera? mark the route up with coloured string so he can 'lead the way' back down? Just try to think of lots of things to keep him excited and interested. Don't be disappointed if it's too much for him (he is only 3!)
I think Trail magazine had an article this month on taking small children up into the hills.
Good luck!