Depends on the power level of the campaign, really.
A child of a mortal and a god might start out as a strong member of his mortal parent's race: so he'll have 1 level when he reaches adulthood. Give him whatever suits him in his momentary state. 10 or twenty might be OK.
Then give him the stuff D&Dg grants Quasi-Deities, say of DR 0. He'll have to earn the rest.
He'll have better ability scores than a mortal, but not the divine default array. Maybe something along the lines of 20, 18, 16, 14, 14, 12. He may even have only mortal-like values, with a single ability score high above the rest (like 25, 14, 13, 12, 10, 8 at the standard array, or +10 to his strongest).
Maybe give him some spells of his divine parent's domains as spell-like abilities 1/day, or grant him some domain powers (or both). A weapon focus and proficiency with the deities favored weapon seems appropriate for the son of a war god.
His type would be Native Outsider, like the Planetouched from FRCS.
The representative of a god could be a proxy, see D&Dg about them. Or, you make the same as above, but he'll be a real outsider, have 20 levels, maybe even some more, DR 1 (for real demigod rather than hero-deity) and the full divine stats (or a higher array than for godsons, maybe 25, 23, 20, 18, 18, 18). He'll get the usual point to rise one stat every 4 class levels, as usual, and another one for his DR1. Full fledged ability scores for a god are 35,28,25,24,24,24 +(DR and Class Levels/4) points to rise.