Hi, My Son Is Almost 6 Yrs Old And On

hi, my son is almost 6 yrs old and on the autism spectrum, but high functioning. He constantly bites his plastic toys and sippy cups and eats small bits of plastic. I am concerned these could be causing some of his issues and want to know if there are tests for this? Could they be building up toxins in his body? He is acting out more in school lately and having more cramps in his hands and feet. We give him vitamin supplements everyday as the nutritionist advised.