My twin girls will be 5months next week and like your son, they put everything in their mouth including feet. I've noticed that one of them when in the exercise-saucer, she's bites on the toys and has it in her mouth. She gets extremly frustrated also.
What we've done is given her a teether (although i can't tell whether she's teething already) or a soft teether rattle (cloth material but it's a teether)...she chews on this forever in replacement.
As for activities, they rotate from a bouncer (where they hit stuff and practice grabbing), exercise-saucer and then the floor for tummy time.