Hello, I'm Dr. Deb and I'll do my best to help you today.
I'm sorry for this concern for your dog. It sounds as if he may be experiencing some discomfort from the injuries he sustained from the attack even though there are no obvious open wounds.
In some cases, injuries to muscles can be as painful as open lacerations or wounds.
I should mention that if he is was grabbed and shaken by the other dog, then sometimes the skin is pulled away from the underlying tissue such that the damage may not be obvious for a few days.
If your dog isn't vomiting nor currently taking any steroids or non-steroidal anti-inflammatory medication, then Aspirin can be given which may help with his discomfort and with any inflammation which might be present. The dose would be 10 mgs per pound of body weight twice a day, with food to avoid stomach upset. My only hesitation about its use would be that it might interfere with what your vet would want to prescribe if he has to be seen but several doses should be fine.
If he continues to act painful or uncomfortable after another 24 hours or so, then a vet visit might be prudent.
I hope this helps and that he's feeling better soon. Deb