It depends upon whether or not he used a vehicle or not. If he was on "probation" as opposed to "deferred adjudication" he will have an underlying sentence which will determine the full amount of jail or prison time to which he can be sentenced.
In addition to the punishment he may receive if his felony probation is revoked, he could face county jail time of 0 to 180 days jail and up to a $2000 fine on the new evading arrest charge if it was a Class B Misdemanor (evading on foot) offense. If he evaded using a vehicle, it would be a State Jail Felony offense and he could face time of 180 days to 2 years in a state jail facility and up to a $10,000 fine.
As far as what can be done to help... I strongly suggest he hire an attorney (or apply to have one appointed to represent him if he is indigent) as soon as possible.
Although my intent in answering this question is to aid you in the legal process, my answer does not establish an attorney-client relationship in any way. You should seek the advice and counsel of a qualified attorney in your community to evaluate your legal needs and to advise you. No Attorney-Client Relationship is created without the specific intent of both parties.
The car theft is a felony theft charge. He probably got deferred for it and it was probably a state jail felony (level is based on value). If so, then if adjudicated he is facing 6 months to 2 years in prison. (If straight probation then max is the number of year that was probated.)
Regarding the evading - it depends on if it was on foot or in a vehicle; if on foot if he has a prior for the same offense and if in a vehicle if there were injuries. Here is the relevant portion of the evading statute:
(b) An offense under this section [evading] is a Class B misdemeanor, except that the offense is:
(1) a state jail felony if the actor uses a vehicle while the actor is in flight and the actor has not been previously convicted under this section;
(2) a felony of the third degree if:
(A) the actor uses a vehicle while the actor is in flight and the actor has been previously convicted under this section; or
(B) another suffers serious bodily injury as a direct result of an attempt by the officer from whom the actor is fleeing to apprehend the actor while the actor is in flight; or
(3) a felony of the second degree if another suffers death as a direct result of an attempt by the officer from whom the actor is fleeing to apprehend the actor while the actor is in flight.
A third degree felony carries 2 to 10 year in prison while a second degree felony carries 2 to 20. The time given on the two cases would run concurrently.
He could also just get jail time if the evading is a misdemeanor and then be continued on his felony probation.
He needs a lawyer.