Hi SusieR,
Flat headedness is a recognised problem called plagiocephaly. My baby had a flat head as from birth he always slept with his head on one side. This is often common with large or tall babies and is more often seen in boys than girls. Theory is that when squashed up inside the womb, the baby's head is not straight and ligaments in the neck become longer on one side than on the other. This is why baby will favour sleeping with head on one side.
If caught early enough, the problem can be corrected by doing some simple exercises with your baby to help lenghten the shorter ligaments on one side.
Do not believe what many Dr's and HV's say about it getting better by itself - it won't. You need to take action to sort it out. I repeatedly went to 2 GP's and 3 HV's and a paediatrician who all said I was making a fuss about nothing. It's a relatively new /newly recognised condition and treatment is only offered by a handful of NHS trusts in the UK, most don't recognise it at all.
In the end I contacted a charity called headstart4babies. My baby was 10 months old by this time with a really flat head that also caused his forehead to bulge forward on one side. My baby had what's known as "helmet therapy" - it's a plastic helmet with a foamy interior specially moulded for your baby. As his head grows, the helmet applies gentle pressure that pushes his soft skull back into a more symmetrical shape.
We were warned that the therapy might not be very helpful as ds was quite old, but the results were really good - not perfect but massive improvement. DS had to wear the helmet for about 16 weeks, 23 hours a day.
As your baby is still so young, you maybe able to avoid helmet therapy and just do the exercises that will encourage him to sleep with his head in different positions.
Check out this website:
\link{http://www.headstart4babies.co.uk}
and this article on the BBC website:
\link{http://www.bbc.co.uk/health/conditions/plagiocephaly2.shtml}
and there are other websites that can help if you type plagiocephaly into google.
Really hope you manage to sort something out for your ds, but don't just leave it and trust the GP like I did...
Raggybaggyx