Yes! There are many different forms of bpd. Some people experience the highs and lows, but it's better to imagine mood as a continuous scale of really low to really high- and they can be anywhere on that scale, really low, only a little bit depressed, normal, a little bit high, a lot high. You'll have to learn more about his unique case to know how the disorder manifests in him.
Plus, with medication, most people who have bipolar disorder are able to maintain normal mood levels.
You are wise to question as most cannot control the swings in their moods. Meidcation is the traditional apporach yes.
However, in all cases of these stress or mood disorders, the emotions/moods are in fact "out of order". In most cases there is an old unresolved emotional trauma that has never been properly dealt with. A natural holistic apporach that would engage complete healing and lasting change would be such as Flower Essence Therapy.