Owenski

Sounds to me weither you built it or not he'd struggle to return it simply due to the time elaspsed since purchase.
Name and shame the shed company please because I'm looking into one and wouldnt mind avoiding this firm.
Did he buy from them direct or from a re-seller?

RE the shelves:
hmmm its tricky, if I were putting a shed up for a mate and found shelves I'd assume they wanted the shelves fitting too so would have just done it. But for a paying customer who's bought something against my better judgement which turned out to be a nightmare of a job to construct and then had additional items in the packaging I'd be inclined to think he's pulling a fast one and tell them it'll be extra for the extras, especially as at this stage it looks unlikely you'll be getting any return business from them.

The guy sounds like a jobsworth, we all know someone like this and they're often a solitary person too stuck in their own ways to take advice which contradicts their own thoughts.
Did you get the fee for the labour, has he actually paid you any of the £250?

If you can see any reason in the bloke, anything which indicates he might be fair enough to reach an amicable resolve. Then I'd suggest you both square up whats owed right now, then take "putting it right" from there on out. I'd insist you cant be accountable for the shed after its built as its not a product you're personally familiar with.
No more than if he took his own brake pads to a garage and asked them to fit them, it simply doesn't work that way. If he cannot appreciate that then I wouldn't touch it, at least as it is now its un-fettled which means nothing you could have done could be accountable for any failure to its structure.

oh and +1 on the getting onto the shed company, get them informed of all this loss of earning potential damage to your reputation etc.