You could look at his Sensory Processing Issues. A lot of times kids with ADHD have sensory issues that impact their daily functions, loud sounds, easily startled, tastes textures, and swallowing, visual issues with bright lights, or fluorescent lights, or to much clutter or visual stimulation they can’t sort out what they need to look at. You can also look at if the spitting is possibly a tic. I have children with Tourette Syndrome, and we have had a spitting tic. I set boundaries of where it was okay to spit, in a cup, water bottle, garbage can or tissue, sink or toilet. It wasn’t okay to spit on ourselves, sleeves, or on the ground because others will step in it, and it grosses them out. We discussed the health issues with leaving our germs around and how we can manage our tic or sensory need to spit, by doing it politely so people wouldn’t be grossed out.
I would reach out to your doctor, and see if maybe if your son is on ADHD meds this could be a compulsive side effect, to the meds, like the metallic taste, or plastic taste. If it is a tic, it can also be a side effect to stimulant medication. Most times they bring out tics the children have. This is what happened for me personally when I went on ADHD meds even as an adult. I had tics manifest, but they aren’t inhibiting me in any way so I can live with the side effects. My kids with Tourettes can’t take stimulants because it makes their tics worse. We take other meds to manage the symptoms. Sometimes the side effects are worse than actually dealing with the ADHD its self. It’s a personal decision you have to weigh in your situation.
It if is tics and not related to meds, you will want to see a neurologist to see if there is another diagnosis to consider with the ADHD.
I hope this gives you an idea or resources to pursue.