My guess would be teething. Does he seem to be drinking as much as usual?

If it's dehydration, you will usually see other signs as well as dark urine. Fewer wet nappies for one thing, probably fewer dirty ones too, he will either scream a lot or sleep a lot more than usual, his eyes will lose their brightness, and his skin will lost it's elasticity (ie if you pinch it, it doesn't bounce right back). Also severe dehydration could sink his fontanelle. If he has these other signs, then seek medical advice immediately. But dark wee is common with teething so that would be my guess.