Saline solution is the first thing I'd try. Use either a product called Simply Saline, available as an OTC, or make your own. I am including a recipe for making and using your own saline solution.
Alternative to antihistamines: 12 oz sterile water (put water into microwaveable container, bring to a boil)Add 1 teaspoon of sea salt, and ½ teaspoon baking soda.
Mix well, cool to lukewarm. Using a baby nasal syringe, fill each nostril and hold for 2 minutes.(note: lying on back across bed with head hanging over side is a good way to get solution deep into nostrils and keep it there for the two minutes.) Have plenty of paper towels handy to catch the liquid when getting up. It is best to go into the bathroom to allow the liquid to drain into the sink. Blow out of the nostrils and suck through the palate to get the mucus out. If this is done regularly, sinus problems should be kept at bay.
I know some of this sounds rather 'yucky' but it does work. Also use the same solution as a throat gargle. At age four, it may not be easy to teach your son how to gargle, but even if you can get him to rinse his mouth with the solution, that may help. If he happens to swallow the solution, no harm is done.
This solution helps keep the mucus, that can accompany any number of ailments, under control. If there is no underlying problem requiring treatment, using the saline may resolve the problem completely. If there are other issues, it will at least give your son's body a better chance to work to heal those problems.