That is good that he is happy either way. Is his relationship with his sister equally positive and healthy either way? Sometimes persons with ADHD may not have the self-awareness to realize that others are distressed, or not in healthy interactions with them, so that the apparent happiness comes from obliviousness, rather than truly being consciously happy with the situation. It may be that he is genuinely happy, and so is his sister, even with all the uncontained energy. (My unmedicated bouncy person is just about deliriously happy all the time, and, though there are moments when it can be exasperating to those in the vicinity, more often than not, the happiness is so contagious that you can't help but succumb. So I'm saying that, from personal experience, I know it can happen.) It may be that he is happy at the expense of other people, in which case it's not a well-founded happiness. From here, it's not possible to tell which of those is more accurate.
And outdoor activities are extremely good for persons with dysregulated attention. The natural world provides a much higher level of stimulation, but of a different kind (it allows for passive-but-engaged attention), than man-made environments do, which feeds the stimulus-seeking brain. HIs brain is also seeking stimulus on multiple levels: both from an ADHD sensation-seeking aspect, and from a GT intellectual/knowledge-seeking angle.
