Normally they will shed their last deciduous teeth (baby teeth) around 6-7 months of age.
We can see retained deciduous teeth that do not shed normally because the adult tooth came up next to the deciduous tooth instead of directly below it. Thus failing to push it out.
These retained teeth should be removed or they will increase the likelihood of dental problems in you dog.
Does this answer your questions? If not please let me know.
Chewing in dogs is not always related to teething.
Many times they will outgrow this phase.
Try and provide 2 or 3 specific toys that he is allowed to chew on. Too many toys can be distracting/confusing.
Every time you find him with something he shouldn't be chewing, tell him no firmly, make him give up the object then replace it with one of his toys that he is allowed to chew.
If he is alone in the house you may want to crate him until this phase passes. Their is great danger in chewing electrical wires and such.