Shan just finished 5 rounds of carbo and I saw the same thing you mentioned each time.  She had very few symptoms after each of the chemo treatments (eating/drinking/elimination all normal), but she always seemed a little lethargic for the first week after each treatment - maybe even a little depressed for a day or two.  Every time though, by the second week after treatment she returned to her same rowdy self.   The one symptom Shan had after surgery and during chemo that really got me worried was some repeated bouts of super fast breathing/panting.  It has gradually tapered off and happens only rarely now.  Vets couldn't ever give me a definitive dx, but best I can guess I think she really was wearing herself out with her rowdy lifestyle on 3 legs that weren't quite ready to take on the task. So I wouldn't doubt that some of Jackson's "laziness" is just his body telling him to take it easier until he's adjusted.  I don't think you should be too worried.  When he is more fully recovered from the amp, you may be able to "jolly him up" after the chemo sessions with toys and games he likes - that seemed to work for Shan after she had fully healed from the surgery.  Best of luck to you and Jackson!