Ok, don't shoot me coz I think it's not strictly by the safe-sleeping guidelines, but I think it's ok...

With both of mine that have preferred to look one way and have developed flat spots, I have 'chokked' them slightly, both when sleeping and when playing on the floor. (If you weren't happy with the sleeping situation you could just do it during playtime). Just put a rolled up baby blanket under their shoulder and hip (but not up near their face so even if they do turn their head to the dominant side, there's nothing there for them to squish their face into ITMS). It's only a very slight elevation (say, 2-3cms) but it's enough that it makes their head turn the other way. I've done it with DS 2 for the past week and his head's evening out again quite quickly. I think it might be helping to stretch the stiff bit in his neck out too as I've noticed he's looking both ways more when I pop him on the floor without the chock.

You could also get him assessed by a peadeatric osteopath or physio who might be able to loosen up his neck as they often look one way because it's a bit stiff.

Finally, sometimes they prefer to look toward you if the cot is next to your bed. If he's looking at you, try switching him so his head's at the other end of the cot.