Apart from teething symptoms does he appear to be well? Plenty of wet nappies, good skin and muscle tone, alert, etc?
Is he BF or FF? Breastfed babies tend to put on weight unevenly, so it's best to weigh them once a month or so to make sure there is some weight gain. Some months may be more, some less....as long as there's weight gain over time it's good
If you are worried then best to take him to see a health practitioner