S.T.
My son has ADD and when he was younger, ADHD. There's no special care to be taken. Just know that his attention span is shorter than many kids, and he will have an abundance of energy. My son only take medicine when he's going to school as i try to minimize chemicals in his body - so when he's at a friend's house on the weekend he hasn't taken medication. I try to keep his food as natural and additive free as possible too - seems that some chemical additives - expecially artifical colors - add to the hyperactivity.
Many kids with ADD or ADHD are very smart and lots of fun to be around becuase they're little firecrackers. So as long as he's got somewhere to stretch his legs, run and climb he should be fine. If weather permits encourage outdoor play - even when it's cold - as long as it's not raining I always pushed my son outside. If weather doesn't permit maybe put on an exercise TV show and encourage the kids to "work out". And give hime stuff to build or do with his hands. ADHD kids generally need to keep their hands busy if they are sitting - legos, building, magnetic toys, drawing,
You are being a blessing to his family and he will have fond memories of times spent at your home - and God will honor the way you're blessing his family! You go mama!