If its teething then maybe try him with a little calpol? If he's bottle fed, then you can add it to his bottle, it'll take the edge off the pain for him.
Are you using anything esle for teething during the day? Powders etc? If not, and its causing a lot of discomort, then Ashtons Powders are good, we used New Era tablets with Lib when she got bad with teething in the early days, now she's same age as you son, and has two teeth so is happier now they've broken through.
She did go through a phase just a few weeks ago though of waking up in the night, when she'd previously been good at night, it lasted a few weeks, now she sleeps through 5 nights out of the week
Edit: Just thought, what we started to do was give her a heavier tea as well, so she now has prridge/ weetabix for brekkie, and a bottle, then some veg followed by a petit filous with fruit for lunch, then at tea time, we give her meat & veg, and either fruit custard/ petit filous with a rusk broken up in it, and some finger foods like rice cakes/ blueberries. Thats at about 6pm, she goes up for a bath at 7pm. She started sleeping better about a week after we introduced that. Not sure if that is any use?