It's tough whenever anyone has a history of "turning on a dime."
It can be like walking on egg shells. It can be dealt with if we know it can happen at any time and can accept this, for however long it goes on. I think this feature may be very difficult for the person to control without some professional intervention. Some people cannot see that they turn so quickly, they seem to lack insight into this, even when people around them talk with them and try to create more awareness. It's much the same as trying to control impulses. (If he is aware of this, I am guessing he possibly feels regret.)

The lack of motivation can be from frontal brain changes and/or from depression. I am having good luck with Wellbutrin SR (bupropion SR).
I was feeling very drained of energy since my concussion. Very severe blah and useless, which was making the situation even worse. Wellbutrin has helped to give me some energy (both physical and cognitive energy), enough to override the formerly overwhelming sense of "inertia." (I'd felt so useless and so guilty for not being able to get beyond the major inertia.)
Everyone around me has noticed a difference and has commented.

I am more active during the day and evenings now, and sleep much more soundly because I am truly physically tired. I am feeling more hopeful, less guilty about what I can/cannot do and am doing things I enjoy with the extra energy. I have always like to have fun! I am back out there, having fun with hubby and with friends/family. I still have limitations, yet there is an improvement. We are looking for "Progress, not perfection."

So glad you are finding interests and pursuing them!
Yoga and meditation can be so helpful!

Keep on taking great care!

Warmly,
DejaVu