Assuming Fort Bend still wants him, they'll get him first (if it's another felony with decent evidence, or something with a victim, I'm sure they will, but if it's a criminal trespass , misd. marihuana, or something like that, they may decide it's not worth it with him going to TDCJ anyway, and drop their case). He'll have to do at least 2 years for the Agg Assault, since it's what's called a "3g" offense (aggravated). TDCJ will pick him up whenever both counties are done with him.
On any aggravated case, a person must do a minimum of 1/2 of their time - that is the first time at which they even become eligible for parole. So, he must do at least 2 years before he is even considered for parole.
Ft. Bend will likely pick him up before he goes to TDC, although I have seen many situations where that did not happen. (If TDC picks him up, he needs to make sure Ft. Bend has a hold on him while he is in TDC and if not, that he keeps writing to them demanding that they come get him and let him take care of his case. If not, then when he is released from TDC, he will be brought to Ft. Bend and if there was not detainer against him, he will get no credit. I've seen it happen.)