Hello there. I do not know what happened, but for some reason, your question did not appear in my list until now. There evidently was some technical issue, but it appears to be resolved.
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The E-2 status is valid for 2 years at a time, but it can be renewed indefinitely. So yes, he can continue to extend his E-2 status, so long as he continues to be eligible for it.
Note, however, that the E-2 is a non-immigrant status. Therefore, he needs to maintain non-immigrant intent when he enters the US and as long as he is an E-2 status holder. Therefore, if he is ever asked about his mother's petition, he should be prepared to provide evidence of strong ties to his home country, to show that he maintains non-immigrant intent. Evidence of property outside the US, family ties outside the US, community ties outside the US. They may not even ask for this stuff, but it is good to be prepared.
Also, when it is time to apply for adjustment of status, he will need to file form I-508 along with his adjustment of status application. That is years down the road, though.
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