I don't recall any particular feeling of low energy when I (we) began low carb. Your husband has so many different things going on,
reducing carbs
low calorie diet
raised blood sugars
blood sugar levels dropping, possibly faster than his body is keeping up
recovering from shingles
a weekend of decorating
that it would be impossible to say exactly what was making him feel a bit slow in the morning. Who knows, perhaps he is fighting off another virus.
Some/many people do feel a bit different, one way or another, when they start low carb. In our case, it was a sudden rush of joy and energy, nothing negative. If it's not so good, sometimes it is lack of salt in the diet, which can be easily remedied.
My recommendation, stick with it, things are getting better, we all have low energy days from time to time.
Sally