How could a warrant for aggravated sexual assault be placed on someone if they have been out of the country for 1 year?

It's unclear from your question why a warrant was issued. It could be for violating his probation on the original charge or because they believe he has committed (or committed in the past) another offense. A probation violation can be as simple as failure to pay fines and fees, or more serious, like a failure to report or being arrested for another charge. It wouldn't have to be for having contact with the alleged victim.

If you are asking if he can now successfully claim innocence on the old Aggravated Sexual Assault of a Child case, the question is probably not. He plead guilty and to get a new shake at the charge he would have to show that his lawyer was ineffective or that there was some other serious procedural defect in his guilty plea. It is very difficult (but not impossible) to prevail on this sort of post-conviction claim, especially given that his case was over about 3 years ago.

You can try talking to some local criminal defense lawyers in the Killeen area about his case(s). They can give you advice about his chances and how he might fight the charges.

Disclaimer: This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.