Tylenol is not safe for dogs at all and should never be used with them. Most over the counter pain meds are not safe at all for dogs with only aspirin being moderately safe.
If this almost felt like bubble wrap crackling this could be a sign of gasses under the skin.Its not apt to be the spleen. You can see the anatomy and where the spleen is in this diagram
http://www.vetmed.wsu.edu/ClientED/anatomy/#Dog
Sometimes this is due to infection, sometimes due to an injury that let air under the skin and sometimes it can be due to a lung issue.
I'd plan a vet visit for him if you can still feel this over any part of his body.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics