The tongue is your baby's primary sensory organ, which is why this is happening. Your baby is trying to explore his world, and just as newborns find their way to mama's nipple with just their tongue and sense of smell, this is his way of finding his way around the world.
Even though his sight and hearing now have context and meaning for him, the tongue is his tried and true way to gather information. "What is this? Is it food? What sort of texture, taste, density (the tongue is a strong muscle) does this thing have?" These are the questions his tongue is solving for him, and how he is navigating the world at present. I mention this because he might find placing things into his mouth a biological/intellectual need which can positively impact his development, so here's a place where I would avoid getting into a power struggle, and instead offer toys safe for mouthing.
He'll get older, rely more on his senses, begin to discern that some things really truly do taste gross, and these experiences will motivate him to *stop* putting things into his mouth. He'll also get older and listen to your reasons for this around 3 and 4. Kids as old as 2 and 3 eat pennies, so it's good to have really reasonable expectations as to what is safe to have around. For example, my preschool group is always supervised with marbles or other small objects, because I observe that subconciously, they are still putting things into their mouths from time to time. And I never give a young child anything they can't put into their mouths. It just happens.
If you are looking for natural toys that are safe for babies to chew on, Nova Naturals is a great company to start with. If you google "waldorf toys", you'll also find some very sweet, wonderful toys for kids. You don't have to agree with the entire Waldorf philosophy to appreciate that these toys are safer for children than the BPH chemical plastics that are offered in the mainstream.
I liked your anecdote, too, about your son closing the book when he was finished exploring it! I'd also suggest cloth or board books for this age. (Once again, online searches will get you the best results.) It also sounds like he's doing a lot of gross motor development, with the wiggling from place to place.
At this age, too, even non-emergent teeth are beginning to move around and become uncomfortable, and mouthing/chewing is one way children cope with this. The pressure on the jaw/gums feels better. This is also a big reason that very young children bite.
For now, I'd encourage you to find toys that he can safely mouth, gum and drool all over.
Best~ and have fun with your baby!:)
H.