I take it you mean he is 170cm tall.
By my calculations that makes him 68 inches tall or 5 foot 8 inches (5'8"). That is a perfectly acceptable height for a mature man. I know several men who are around this height or a little smaller. Anyway, he may not have stopped growing yet. Don't worry about it, and don't make him self-conscious about it.
A good diet and normal activity levels will help him achieve his natural height, which is controlled by his genes. Drug therapy etc is only available for extreme cases, and can be very dangerous.
Viv 8)