Since the back-to-sleep campaign, heads have got flatter, so to a certain extent, more normal......
We bought a special pillow for our DS4 when he was about 2months, as he tended to turn his head slightly to one side so not only was it flat at the back, but also uneven. The pillow was called a butterfly pillow, is safe from birth, looks like a donut. There is also a mattress with a grove in it, which is said to prevent flathead, but we already had a mattress so just used the pillow, and results. Were visible within 3weeks... He's 11 months now, and is still a bit flat, but it's even on both sides.
The older the baby is, the longer it takes, but still worth trying the pillow or mattress, don't listen to the HV when she says no pillows for babies, as breathable ones designed for babies are fine. She may also try to convince you that it's fine, the hair will cover it etc, but go with your own instincts.
Try to keep DS2 of his back as much as possible- sitting, on side, on front, as the flat head is a direct result of the huge amount of time our babies now spend with the back of their heads pressing against a surface, cot, car seat, pushchair.
I am no fan of baby walkers, but bought DS4 similar thing, bouncer activity centre with no w heels and made sure he had 15 or 20 min in it 2 or 3 times a day as soon as his back was strong enough, 5.5months I think, until he could crawl and sit up himself.