Hi A.,
It might be that your son is teething, but tylenol and teething drops might help but won't necessarily make the pain go away. Also, unless he has a fever of over 101 F, it's not a great idea to give tylenol to a child under a year old (despite what your ped. may have told you) -- I only realized this after having given tylenol to my daughter after shots, for teething, etc.
It might also be that he's about to accomplish some big developmental milestone, either physical (crawling? rolling over?) or mental.
If it's either teething or some developmental phase, the good news is that it should resolve itself soon.
Have you have any changes in your lives as a family recently? Gone on any trips? Babies can really pick up on stress. My daughter slept a really good (5-7 hour stretch) from when she was 6 wks. old to about 3 mos. and then she started waking up every 2-3 hrs. Right around this time, we were headed to visit my parents for over a week (and I think she needed us to move her 10 PM bedtime to more like 7). So trips, remodeling, or other sources of stress/change in your life can affect your baby (and his sleep!).
We did a lot of unintentional co-sleeping while my daughter was going through this (she ultimately started sleeping 12 hrs. straight around 10 mos.), and eventually she was only waking up twice (and then once). When she was around 8 mos., we just ignored her 8-9 PM wake up and she did fine. Then around 9.5 mos., we ignored her in the wee hours of the morning. But I know what you mean about crying it out. And it works well for some babies (and some parents) and not others.
So, I guess I would suggest that you stick to your routine and do your best and hope that things will get back to normal soon. Whatever happens, good luck. Try to stay calm -- you're doing a great job!
Best,
R.