J.M.
Heat rash is usually more common in areas that are covered with clothes, or in creases... Where sweat isn't allowed to flow freely. (I know a lot about heat rash, since my DD has overactive sweat glands, which makes her REALLY prone to heat rash...)
If she has a rash on her cheeks, but not on her chest, neck, armpits, back... pretty much anywhere people sweat a lot, but is usually covered, I doubt it's truly a heat rash. It actually sounds a lot like an issue my DD had, around that same age... She had little bumps (some were red, some were just bumps... sometimes the whole area would get a little red, but sometimes just the bumps...) that were mostly on her cheeks, but over time it spread down her chest a bit and onto her back. While it looked JUST like the heat rashes she gets now, it turned out that it was CRADLE CAP! I had the flakes and such under control on her actual scalp, but somehow it spread down to her body. I asked the pediatrician about it, and she prescribed a mild steroid ointment that I put on my DD 2x a day. It cleared up the rash within a week... It tried to come back every once in a while, so I would still have to use the ointment about 2-3x a month to keep her skin clear. She DID outgrow it around 4-6 months. :)
Like LittleEsmom said... It could also be baby acne or eczema... They look very similar, and are very common in babies.
THAT SAID... IF you still think it's heat rash, and are looking for ways to prevent/treat heat rash...
1. Dress your DS in loose fitting clothes. Heat rash is caused when the sweat gland gets clogged, usually because the sweat is held against the body by clothing. Loose clothes will allow the skin to breathe. I actually don't have my DD wear any clothes at all, other than underwear, when we are home... she only gets dressed if we have company coming over or we leave the house.
2. Before getting him dressed, use a large makeup brush and brush him down with a cornstarch (not talc... talc can get into his lungs and cause issues) based baby powder. The powder will help absorb the sweat, allowing his pores to breathe.
3. Throughout the day, like maybe at each diaper change, wipe him down with a cool cloth. This will not only help lower his body temperature and slow down the sweat, but it will also help clear any sweat away from the glands, preventing the buildup that will plug them up. After the wipe-down, re-apply baby powder again.
4. If he already has heat rash, some baking soda in his bath water will help a lot!