Are these circuits of the "snap circuits" variety, or something else? If he likes to put things together, maybe you could give him a complicated puzzle to work and videorecord/photograph him working it.
You could similarly try giving him different types of building sets, and see what he comes up with. Some ideas might include wooden or other basic blocks, wooden train sets, Legos, Capsela, Zome, ZOOB, Erector sets, etc.
One thing I've done with my older son in the past is give him fun challenges. For instance, one day a while ago I told him that I wanted him to use all of his train set pieces (which is a lot) to put together a single track in a small L-shaped part of our living room, without leaving anything out, and where every part of the track had to be reachable from the station house. It was a fun challenge for him, and he went right to work putting together a multilayered track, which took him a little while. I set this task for him just because I thought he would find it fun.
I mention this only to get you thinking about ways you might be able to encourage your kid to produce something recordable as evidence of his gifts, while still keeping it fun and non-threatening for him. Set a goal that will require some level of complexity to achieve, and see what he produces.
