Learning loads from reading all the threads on here, and realised that what I thought was a DS only problem isn't!
DS has ALWAYS had to have something in his mouth, including trip to ED at 18months after he swallowed a drill bit. Anyhow, even now at the tender age of 8 he is terrible for putting things in his mouth. He wrecks clothes by chewing the sleeves - not just the end, he eats them up to the elbow within weeks. We tried putting the 'stop nailbiting stuff' on the ends but that led to him chewing holes out the chest area instead.
Otherwise, he chews on flowers, paper, glue, anything really that he can get his hands on. I regularly catch him trying to lick water off wet objects (fencing, walls, puddles etc).
His old school brought in a 'rule' that you mustn't eat stones out the playground after they found him chewing the gravel instead of doing PE.
I think from reading the other posts that this may be something (sensory disorder?) that can go hand in hand with Aspergers (currently waiting assessment).
So, what do you do about it? In terms of chewing he is worst when he worries about things - so school jummpers, school coats (playtimes) and pyjamas are the worst affected clothes so it costs me a fortune.
Also worried he is going to catch something nasty when eating stuff outside - I have tried to explain he could get worms, but that doesn't really work for very long.