Welcome! I am Dr. Altman, a licensed veterinarian and I am happy to answer your questions.
I unfortunately would advise a return veterinary examination
I would also advise taking a video of his movements and what you describe with difficulty getting up, moving about, lethargy so the vet can see what you describe because often when they get to the clinic their adrenaline kicks in and they seem perfectly normal to us!
Glad to hear his basic vitals are normal but it does sound like something is going on and a basic blood panel, evaluation of the video of his behavior at home, and a repeat examination to include a complete neurological exam is warranted
I would also advise using a one to ten scale (ten being not moving at all) tonight and again in the morning to help your vet evaluate the severity of his lethargy and energy level. These things really do help when he is fairly normal on presentation
Please let me know if you have any additional questions. I apologize for the delay, I just signed on and saw your question so hopefully my information is helpful to you