gamegrene.com

I have no idea what your setting is like, so some of my ideas may not be consistent with your setting.

There's two major questions to consider here, the first being "What will be good for the story?" and the second, "What does my player really want here?" My general advice would be to give the player what he wants (whatever that is), so long as it doesn't clash with the story being told or with the setting you're playing in.

I do have some thoughts about how I would handle this specific situation within a generic D&D world. First of all, in most fantasy settings, the likelihood of sex between a dragon and a humanoid is already exceptionally rare, and most of the time the humanoid has no idea that their partner is a dragon. The probability that your PC will find an adult dragon that would be willing to mate with them is pretty slim, never mind the potential risks of making sexual advances on such a powerful creature (I imagine dragons have a pretty low tolerence for sexual harassment!). Add in the risks of miscarriage, stillborns, and the general poor quality of non-magical medical care, and the likelihood of such a child coming into existence is so miniscule that you probably wouldn't have to worry about it.

However, let's assume that such a child is conceived. The only reason that a dragon and a humanoid could produce a child in the first place is because dragons are such powerfully magical creatures, and the magic somehow overrides the usual restraints on cross-species reproduction.

The gender of your PC would be the deciding factor for me, here. If your PC is male, then the child would be lain in an egg by a dragon mother, and I would say that the mother dragon's innate magic would highly favor dragon traits over elf ones. In this case, I'd make the kid a true dragon, although he would be obviously smaller and weaker than most dragons of his age, with more of an affinity for a humanoid form. Statistically, I'd use regular dragon stats, and make the difference apparent only in the narrative.

If your PC is female, I'd call the child 3/4 dragon, and just use half-dragon stats for him. Story-wise, he's still more dragon than elf, but because his development isn't being directly influenced by draconic magic, he'd still have elven features as well. If this kid ever became a PC or an important NPC, I'd make him take a couple levels of sorcerer (or some other similarly dragon-themed class) on top of the half-dragon templete and any other class progression he wants to take, to represent his goofy heritage.

Remember that all of this is based on my own interpretation of how things work in a D&D universe, and my suggestions may be worthless, as they were submitted completely out of context. However, I hope this helps.