If they're offering to "take him off your hands" then tell them you appreciate that they are trying to help, but that your stomach is ALREADY in knots thinking about the separation, so they wouldn't exactly be doing you a favor. Surely they wouldn't press the issue if they knew you would be anxious and fretful and missing him the whole time. When my in-laws first came to visit the newborn, they told us to go out on a date, have dinner or a movie together while they watched the baby, and I had a hard time explaining how much I *missed* my baby when I had to work all week and I *wanted* to be with him, *craved* it, even. We compromised and they were DELIGHTED to have him nap on their shoulder while I bustled about the house getting things done.
Yes, you're probably uncomfortable about how they'll be able to handle your son, especially during the road trip, but it's probably best not to mention that. I wouldn't bring up how it would be an inconvenience to *them*, but how much of a change it would be for the baby. He's not old enough for you to explain to him where mom and dad went - no matter how doting they are, it's a huge change, new house, new caretakers, new EVERYTHING. He'll be completely out of his element, confused, and no one will be able to explain what's going on. He just won't understand. Say you just can't bear to be away from him, you would miss him too much.
Of course, if he still nurses at all, (or they don't realize that he has stoppped) you can bring that up. Even a subtle hint about your breasts being painfully full with the baby gone, well... whose gonna argue with that?!? I know my father-in-law would turn beet red and drop the subject like a hot potato - probaby even jump up and pretend he forgot about something in the other room.
If they have the audacity to persist, just explain "Where he goes, *I* go. We're a package deal." Surely you would also be welcome, too.
My son is almost 6, and there is only one occasion which he spent the night with his grandparents. Once when he was almost 3, we were in town for a class reunion that went late at night on a Friday and Saturday. With MUCH resistance and fretting on my part, I allowed them to drive him to their home after we had dinner together at a restaurant. They bathed him, put him to bed (I inisisted that his grandmother sleep *with* him, because he slept with us at home), made him breakfast and brought him right back to us. That happened 2 nights in a row, and that's been it!
(They live far away, though, so it's not likely that he would travel that far without us.)
Remember, you are the mom.
Mom ultimately calls the shots.
And the rest of the world just has to deal with it.