You need to get them out! Most decks are made with "pressure treated wood" or what they call CCA wood. CCA wood is Copper Chromium Arsenate. CCA or pressure treated wood contains dangerous, hazardous chemicals, Including a form of ARSENIC, one of the deadliest poisons known to man. As hands and feet come in contact with the wood the chemicals can either get absorbed into the skin or mouth. CCA has been linked to numerous cases of caner and other diseases with significant % of cases involving children. If you can not get them out, get the doctor to get them out. I use toenail clippers on my children's splinters. It does not take as long as tweezers, but it does take more skin away and sometimes you can get the splinter on the first try.
Splinters do "work their way" out of the body, but it could take a long time to do so and since it is deck wood, I would think he is more inclined to infection, even if it was just cleaned! If all else fails, take him to your doctor and get them to remove them. Good luck!
Mama to Bryce~9, Austin~6, Taylor~1 1/2
Step Mama to Nich~15 Christian~14