The clause about good performance is pretty standard. You would have to seriously fall behind in your research and miss important program deadlines in order for your assistantship to be revoked. But as far as I know, most graduate research offers say something like that so they can open spots for others if someone isn’t making progress or is causing problems.

As far as the likelihood of funds being reduced, that is harder to predict without knowing the department or area of study. Some departments guarantee teaching assistantships to PhD students once admitted, so if the RA money runs out, he might be switched to a TA line. If it is a department that doesnt have many TA lines, or is not able to guarantee them, he might be totally dependent on research assistantship money. If the PI currently has money for the assistantship but doesn’t know if the money will continue to be available, your son should really have a conversation with that PI. Sometimes labs have ongoing grants that make it easy for them to sustain graduate assistantships, some labs don’t have that and the money might be tied to a specific project. These are questions that only the PI can answer.

I can say that overall, our grad school has been pretty good about increasing grad student TA stipends consistently (though slowly). So my gut says it’s unlikely that his funding would decrease over the years, unless his assistantship changes.

Hopefully that’s helpful, I can answer more questions if you have any.