14 weeks is a good age for them to begin eating solid food. Some cats will start to eat on their own sooner, some later – depends on the individual cat and how hungry they are.
I’d give your kitten an oral vitamin supplement if he seems really underweight (looks sickly), but I wouldn’t worry about iron supplements at this time because it’s not necessary. Your vet can recommend one that has no artificial colours or flavours in it for kittens, especially since you’re worried about his teeth rotting out – just check with him what he recommends so that you don’t accidentally get something detrimental for your kitty’s health! *hint*