14 yr old DS insomnia

[trulyscrumptious43]

I realise that most people on this topic will be talking about small children, but my 14 yr old is having trouble sleeping and often doesn't get off til 1am - 4 am.
This morning I went in to wake him up to find he'd left me a note saying he didn't get to sleep til 5.30am and please could I let him sleep in. It's terribly disrupting for school as he often goes in like the living dead and has a rotten day. This has been going on since the summer school term.
Any help or advice? We do the warm milk at bedtime, bath before bed (though not every night, he's 14, after all!) and he reads a book in bed if he can't sleep.