There are a couple of problems with your question. First of all, Aggravated Assault is a second degree felony and punishable by 2 to 20 years in prison (assuming no priors). If your son was placed on probation and served his 90 days as a condition of that probation, then he was ON probation and the Judge could not "refuse" it.
Now, the judge could have revoked it, but I'm confused about the part where you say he "gave 4 years in TDC". A prosecutor does have the ability to file aggravated kidnapping, which is a 1st degree felony and punishable by 5 to 99 years or Life in prison. It is not unusual to see prosecutors upgrade charges on an accused person in an effort to get them to plead out. But, that is not an automatic Life sentence.
He can make the argument that his plea was coerced and involuntary, but a Motion for New Trial needs to be filed almost immediately, and there will need to be some proof of the fact that his plea was involuntary or some new evidence that wasn't available at the time of the plea.
Your son should consult with a lawyer right away. If he wishes to withdraw his plea, he should do so as soon as possible, preferably before sentencing. The longer he waits the harder it will be. Only a lawyer meeting privately and fully advised on the case could offer good legal advice here. This response is for informational purposes only and is not offered as legal advice.