You definitely should not feel quilty for leaving him! Trust me, I watch kids overnight and even for weeks at a time while they're parents are away on vacation or away for business. Just this past weekend I had my 4 & 6 year old cousins of which their parents hadn't gone on vacation in about 2/3 years, so they definitely needed it.
Sleeping is going to be a HUGE issue and it will probably be the biggest concern/issue with you leaving. If at all possible I would get him to sleep on his own prior to your trip. It will make things so much better and safer. What if your in-laws are deep sleepers and wont wake up if they roll over? eek! On the contrary, what if they do not feel comfortable sleeping with him and decide it's best to have him sleep on his own ...of which then your son will feel way to much stress with all that change at once. I would really really recommend that you try to get your son to sleep in his own bed. Start out by having him fall asleep with you or in your arms and then simply move him to his own bed. This would be an easy enough transition that you will have enough time to do prior to your trip. Then your in-laws could do the same... he can fall asleep in their arms or next to them and then they can move him in his pack n play. After the trip if you like the family bed you could always go back to it but for your sons sake it will make things a lot easier if he is able to sleep in his own bed prior to the trip.
Some other important things to remember are to leave his insurance card, just in case. Also type out a letter stating that you grant full permission for your in-laws to seek medical treatment and that you can be reached at ... (phone number). This is the only way that drs. will be allowed to treat your son in case of an emergency. It's one of those things that if you give it to your in laws then they prob wont need it but it's better safe then sorry.
Enjoy your vacation and remember absence makes the heart grow fonder (even when it comes to your children!). Definitely try not to worry. In the end just trust your in laws and know that they will always have your sons best interest in hand. What you are doing is perfectly fine and your definitely shouldn't feel quilty!