I'm sorry to hear about your fellow's seeming random yelps, and difficulty jumping. He sounds uncomfortable.
Since he just seems to be vaguely not himself that often points to musculoskeletal pain. Dogs that are painful will hesitate jumping or climbing stairs and display the sort of behavior he is, either because it hurts to do the activity or because they feel weaker than usual. If he has arthritis then most of the time these pups favor a particular leg.
Unfortunately Beagles as a breed are prone to a problem with their intervertebral discs, which are the spongy cushions between the individual vertebrae in their back and neck. These spongy discs can move or rupture and place pressure upon the spinal cord which can lead to pain, and in severe cases paralysis. Disc pain can be intermittent, and if they move just the wrong way nerves get pinched and the pain can be excruciating and sudden.
Radiographs can sometimes be diagnostic but often early on in the disease process, because the discs are soft tissue not bone, everything will look normal. An MRI is the best way of diagnosing disc disease.
If the dog is painful but has no evidence of paralysis we can try strict rest, anti-inflammatories and pain medications for several weeks to allow healing.
If there is evidence or weakness or paralysis then surgery by a board certified veterinary neurologist, as soon as possible, is indicated.
Ideally he would see his veterinarian. If this is indeed a disc problem your veterinarian can prescribe a steroid or nonsteroidal anti-inflammatory to relieve pressure on his spinal cord and nerve roots, as well as something for pain too, such as Tramadol. And if he is having painful muscle spasms then a muscle relaxant such as methocarbamol as well.
He should be closely confined starting now. No stairs, running or jumping. If you have a crate for him I highly recommend using it. The less he moves around the more comfortable he will be and the faster he will heal. He should go out on a leash to relieve himself. Do not use a collar for him, a harness which more evenly distributes forces if he pulls on his leash is better. You will need to confine him for several weeks, even as he starts to feel better or he may reinjure himself.
Keeping him on the thin side is recommended to decrease stress on his back, but is no guarantee that he won't have another episode. Once a dog has one bad disc the likelihood of another is very high.
If you are interested in reading more here is a link to an excellent article about intervertebral disc disease, its causes and therapy: http://www.petwave.com/Dogs/Dog-Health-Center/Bone-Joint-Muscle-Disorders/Intervertebral-Disk-Disease/Symptoms.aspx
Ideally he would wait to see his veterinarian to take any medication because the prescription medication your veterinarian has for pain will be much safer and work better than any over the counter medications that we take. In fact acetaminophen (Tylenol) and ibuprofen (Motrin) aren't used in dogs because their effective doses are very close to a toxic dose in dogs.
If you cannot have him examined right away and he is still eating and drinking ok, but is really miserable today the only over the counter anti-inflammatory that can be used in dogs is buffered, enteric coated aspirin (like ascriptin). Aspirin does cause stomach and intestinal irritation and ulceration as well as clotting problems so should not be given for more than 2 to 3 days consecutively and should always be given with a meal. If you choose to use it watch for lack of appetite, vomiting, blood in the stools or dark tarry stools and stop immediately if you see those. Do not use aspirin if your dog has liver or kidney disease or a history of a sensitive stomach or clotting problems, or is not eating.
The dose for aspirin is 5mg to 10mg per pound of body weight orally every 12 hours (about one half of a 325mg aspirin for an 18-35 pound dog every 12 hours). Always give with a meal. Do not use for more than 2 or 3 days.
Be aware if you choose to use aspirin and it doesn't help your veterinarian will be limited on what they can give as there must be a 5 to 7 day washout period between different nonsteroidals or nonsteroidals and steroids.
You can try alternating warm and cold packs on his lower back and neck areas for 10 minutes at a time several times a day.
There are other less common causes of back/neck pain such as infections, tumors of the vertebrae or the spinal cord itself or fibrocartilagenous emboli but far and away disc disease is the most common cause of back pain in dogs.
Please let me know if you have any further questions.