I am sorry to hear about Sam's short episodes of collapse with excitement. When a dog seems to lose control like Sam does we worry about 2 things.
1) A seizure which is a loss of conscious muscle control with rhythmic muscle contractions occurring. These dogs may lose urine or stool continence, they are unaware of their surroundings and their muscles tend to be very tense during the episode.They seem fine afterwards, although puzzled about what happened and can be a little tired. This is a link to a video of a dog having a seizure: http://www.youtube.com/watch?v=VSL1_yeKo5o&feature=fvwrel
2) the second possibility is a fainting episode (syncope) or episode of feeling woozy due to low oxygen levels. These episodes can be caused an irregular heart rhythm (either very fast or very slow) such that oxygen doesn't get to the brain and they pass out or get very weak and collapse. These dogs seem to stiffen and then slide to the ground like a wet noodle and have more relaxed muscles compared to those dogs having seizures. They too tend to lose consciousness or collapse, the heart resets itself and they "wake up" and seem normal. Some of these dogs lose urine and stool continence and some do not. They may have minor muscle twitches due to low oxygen levels, but they are not as stiff or rhythmic as true seizures. This is a link to a video of a dog having a quick syncopal episodes. They are usually longer then this dog's: http://www.youtube.com/watch?v=mkj4jREIec8&feature=fvwrel
The irregular heart rhythm that leads to syncope can be hard to pick up on a regular exam as they may only be triggered by exercise or excitement or even sleep (extreme relaxation). And if they aren't stressed at their exam their heart rate may be slow and normal (these are dogs with tachycardia or fast rate arrhythmias) or if they are stressed then their heart rate may increase to an normal level (these are dogs with bradycardia or very slow heart rates).
Both of these "episodes" can be triggered by excitement. If Sam has a history of a heart murmur or heart disease then a syncopal episode is more likely than a seizure. Being a Boxer also makes it more likely that his episodes are related to heart disease. Boxers have a much higher incidence of heart disease compared to other breeds of dogs.
Seizures in older dogs usually signify that there is a medical problem. Younger dogs (6 months to 6 years) are more likely to be diagnosed with idiopathic epilepsy or seizures of unknown origin but in older dogs seizures are usually related to metabolic organ disease such that waste products build up and affect brain function or a primary brain problem (inflammation or tumors).
Unfortunately other than keeping Sam quiet, and avoiding any stress or excitement, there isn't much you can do at home. He needs a physical examination, bloodwork and possibly further testing to diagnose his condition.
If this is heart disease we need to properly diagnose his arrhythmia to know how to treat him medically and if this is indeed a seizure then looking for and treating underlying metabolic organ disease is needed or medication to help decrease seizure frequency and severity if this is related to primary brain disease.
If you could videotape him (many cell phones have the capability to do this) that would be tremendously helpful for your veterinarian as well.
Please let me know if you have any further questions.