My dog hasn't been acting like himself after visiting another dog

[4Boon]

I have a 3 year old half shitzu & half maltese, his name is Boon.  He is usually a happy, excitable and a bit territorial dog.  

5 days ago he spent the day with my brothers Scotty dog.  Nothing too eventful, I thought.  He seemed to be a bit scared when I brought him home, which is unusual...  But I thought maybe he is just tired.

The following day, he doesn't seem to be improving.  The usual "tough guy" is hiding, and not even getting excited when people come to visit.  

So I looked him over (he has a lot of fur), and I can feel laserations on his neck.  So I am thinking my brothers Scotty dog got a hold of him and hurt my dog.  

I never saw any blood, but I can tell he had been injured.  Now it has been 5 days, and he is still not acting right.  He occasionally yelps out of nowhere, which I am assuming his injury is painful.  I have not seen any improvement  in his behavior.

I am not sure what to do.  He is not eating or drinking as much as normal either.  

Since it happened 5 days ago, I can't imagine that it could be life-threatening.  Or could there be something seriously wrong, internally?  

[JanisM13]

After 5 days,besides the pain,it can be VERY
SERIOUS as bites leave puncture wounds
that can close,cause a huge infection so if it
was my dog I would have taken him yesterday!
PLEASE take him to your vet ASAP, infections
can cause serious problems!! AND MONITOR
when the two are together from now on so this
doesn't happen again.
Jan

[Margot49]

Agree with ginger to take him in to get checked.  There may be a puncture wound and that can get infected.  I would not delay.

[ginger899]

I would take him to the vet for a check just to be on the safe side.Tell the vet you are not 100% sure but he could have been in a scrap with another small dog.
If he is yelping suddenly when ot touched, and for no reason , doesn't want to behave like normal, and his appetite is gone then that sounds like he is in pain. A vet needs to see him.