is this just for an 11 year old? he will need some money for snacks - chocolate (both solid and hot/liquid) are a pretty important part of ski-ing holidays. other than that I can't really think what else he'll need.
for ski-clothing try ebay, tk-maxx, and. as you said, begging/borrowing (maybe not stealing ).
he will need jacket - dds' jackets cost about £40 - but is their winter coat - ie they wear them all the time and don't have anything else - too late for this winter but you could maybe get something that will see him through next winter. I;ve never bought salopettes new. got a pair for about a tenner on ebay that have done 4 years service (2 for each girl) and otherwise borrowed - so can't help on price there.
depending on when he's going you might want to get him thermals - leggings and top - I got dds' from Millets - the leggings were on offer - 4 for the price of 3 so I got 4 different sizes - two of which they'll grow into in a couple of years. iirc you;re looking at about £10 for top and similar for leggings - but can end up paying less if you find an offer.
he will need good gloves - you can pay anything but probably about £20-30 for reasonable quality from a good sports shop.
goggles and/or sunglasses (it goes on....) maybe £10. but at that price they might not be very trendy for an 11-year old
socks - look for offers - you can get them for £3-£5 each. he can probably make do with 2-3 pairs.
tops/fleece/sweatshirt - don't get him anything special. you really don;t need wicking, all singing-dancing, £15 or more tops. just his usual t-shirts will be fine.
hat and scarf (best scarves imo are the fleece neck things) - again you can pay pretty much anything but maybe £15 for both - and they'll last a lifetime.
suntan lotion/lip salve - he'll need a diddy tube to keep in his pocket (no need to buy a bigger one too -the diddy one will be enough) - can be quite expensive - I think it's worth buying good quality stuff designed for ski-ing - very high factor, waterproof, for sensitve skin etc and (guessing a bit here) that will probably be £5-8.