Thunderboltkid - a fleecy all in one with vest and babygro underneath should keep him toasty, particualrly with a sheepskin underneath. These ones from Trespass: http://compare.ebay.co.uk/like/160985736539?var=gv<yp=AllFixedPriceItemTypes&var=sbar&_lwgsi=y&cbt=y&device=c&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0&ff12=67&ff13=80&ff14=65(can't find them on Trespass website) are great if you can get them as they have a hood which is good for extra warmth and feet and hand covers which fold back (brilliant for getting their feet into wellies in the morning when they are toddling and for when you want to get them into nightgear but still let them stay up round the fire for a bit etc).
However, Primark and Mothercare are also good for all in ones. I buy them at the end of the season for the next year as the prices are generally slashed.
Have skeepskin (he sleeps on one normally) and have just bought one of those cots of ebay
We're going to Sandy Balls in the New Forest - it's a bit like centre parcs and has an indoor pool so we can take him swimming, and we're hiring some bikes and a little trailer as there are loads of flat bike trails nearby