Repositioning might work- 18 weeks is still quite young.
One of these would help, as he won't be ablt to roll out of it, but you could use rolled up towels under his back/shoulder on the opposite side to the one you want him to use. This has worked for my DS (13wo) but repositioning did not work for my dd (now 3)
I have to say, I was extremely disappointed with sleepcurve mattress- DS still developed plagiocephaly on it which isn't supposed to happen.
We use a goigoi too- for all changes, in pram, baby bouncer, etc. That coupled with just not laying him down much really helps- he is pretty much back to bounds of normality. (no-one has a perfectly spherical head!)
As Soupy says- slinging will prevent it worsening, though by 18 weeks it may well already be quite bad tbh. This could be a good option for you though, as I'm guessing he will be late to sit up by himself, so probably rules out using a bumbo or similar sitter. (sorry- I don't know anything about his condition, so forgive me if I'm mistaken)
Craniosynostosis is very rare, but can be diagnosed very quickly once you are seen by a paed- your GP can refer you.
The misery sounds as if he may have torticollis- GP can confirm/dismiss this, and can give you physio exercises to do with ds each day to improve his use of his neck. You may be able to tell though- dd would whimper if you even touched the side of her neck which had the tort.
I have to say I know several children who had this as littlies, and they haven't grown out of it- it is still noticeable, though less so than when they were tiny. Having had a child with plagio, I am much more sensitive to it though, so I spot people with plagio and brachyephaly all the time- even adults, especially when you're sitting behind them on the bus.
My dd's ears were pretty far out of alignment.