B.P.
We tried supplements earlier when my son was younger but they never made much of a difference anyway. I think it is better to find out if there are food sensitivities. My son had a severe reaction to red dye...we found out because he would always melt down or get hyperactive when we gave him liquid tylenol which had red 40 in it. After we started using dye free, he didn't have that reaction to it anymore.
I have to share that the best way for our son to calm down was for us to use a system of scheduling, using a disappearing clock (let me know if you need to find one), deep pressure/tissue massage/brushing, using a tool book (I can show you how to make one), and heading off meltdowns by having a place where he could go to decompress (earphones/music/special pillow/timer). We also never did the diet either because our problem was actually trying to get him to eat a variety of foods due to the texture, etc. I'd be happy to share any and all that you want to know. My son is 15 now and doing great...so take heart and know that every little step forward makes a huge difference because in the end it all adds up....
Hugs, B.