If I leave money to my married step-son, is it his to keep or does he have to split it with his wife since Texas is a community property state.

If you leave money to someone in your will (or otherwise gift it to him), it is his separate property and his wife has no claim to it provided he keeps that money separate from community funds (i.e., in a bank account in his name and which contains only that money).  Keep in mind that interest earned on seoarate property belongs to the community estate.  The rules can get complicated.   If the amount is significant, you (or he) would be well advised to seek out legal advice.