In the case of the Boxers, a seizure is of course possible. However the vet is the first stop and do get full blood work done which includes a CBC (Blood Count) including platelet numbers. If possible and ultra sound and X Ray would be advisable. There are many cases when their are no outward signs at all and sudden collapse takes place. It may be an internal bleed from a burst tumour and in the case of small bleeds which very often stop as quick as they started, the animal will just get up and carry on. But it will happen again. If you have any doubt get a second opinion . If you are in Canada, contact the veterinary dept at the University of Guelph for advise. There is a reason. somewhere. One thing I notice is missing in the posting, How were his gums, When a dog is in distress the gums will generally, but not always go very pale or even white in serious cases as with my Siberian Husky and this is why we can say, He is very pale, I would also suggest that if an animal COLLAPSES take it without dealy to the vet as it is an Emergency and not a thing that should wait for an appointment. I am not a vet but this has been drilled into me by my vet over the years and I have one that will respond without delay 24/7
Hope this helps a bit , Let us all know the results , its very important to us all, so we can learn too. God Bless and Good Luck.