OP, hang in there. I could have written your post when my son was 18m. He was such a full-on baby, could not be by himself for 5 mins, whinged an incredible amount, and seemed to need soo much stimulation. We just used to go out the entire time, as he could not cope with a morning at home as he lost interest in any toy or activity once he'd played with it 5 mins (tops), and the whinging would escalate to meltdowns. Aged 3-5 months he used to absolutely SCREAM the place down, with the most bloodcurdling noise you have ever heard, for no apparent reason. At this age, even creche were saying "wow. that's a very strong willed little boy".
I can see why you are looking into altern health testing etc. but he might come good on his own. My DS is now almost 2.5. he still throws a humdinger tantie from time to time, but it is increasingly apparent he is highly intelligent and creative but has a v short attn span. But this has improved and (at good times of day) he will 'pretend play' by himself for 10-15 mins. I think they need their imaginations to catch up to their required level of stimulation. E.g. watching him play now, he will use the same toy as a bike, a fire hose, a BBQ, a horse, a skateboard, all within 3 mins! While loudly chatting, singing, and obviously v happy. But a year ago, he just used to get mega-frustrated by e'thing. I used to envy those mums that could actually leave their bub in a playpen for 10 mins to do the vaccuuming or cook dinner - DS would not stay in one for 2 mins! Or the bubs that would play with something contentedly for 10 mins at a time and were just so chilled - amazing! But as they get older, you (hopefully) get to experience the positives of their character that were just hidden before. The one thing we also did, which I think has paid dividends, but has been v hard, is to really limit TV. They have linked TV viewing before age 2 with ADHD, and so (esp b/c DS had such a short attn span and Dh has a lot of ADHD traits too) I was strict abt this. I have relaxed it a bit since he has turned 2, but I think it helped, as it is the wrong sort of stimulation for this kind of child and can easily wind them up more.