He isn't his usual ______ self
means that a person usually has a certain trait, but, for some reason, they are not acting that way in the moment.
For example, if someone is usually cheerful and friendly, but they are acting grouchy, you might say:
She isn't her usual friendly self today.
On the other hand, if someone is often gruff and cantankerous, but for some reason was behaving friendly yesterday, you might say:
Did you hear Bob yesterday? He wasn't his usual grumpy self.
So, in the case of your sentence:
He wasn't his usual ebullient self.
That means he is usually energetic and outgoing, but for some reason wasn't acting that way at that time. Perhaps he was ill, or maybe he was just having a bad day.
The phrase usually refers to a short-term lapse in regular behavior, but I suppose it could be used for a permanent change. For example, we could say:
Bob just didn't seem energetic anymore; he wasn't he usually ebullient self. So, after a couple weeks of feeling exhausted, he finally decided to go see a doctor, where he learned he had Stage 3 cancer. That was such a tragedy.