Not only am I an Audiologist, but my son had chronic ear infections as an infant (despite being breastfed), and got his first set of ear tubes just before his first birthday. Fever is the main sign of infection. You haven’t mentioned fever, and the apparent pain when sucking could be an oral or throat issue. Try to look in his mouth for signs of teething or any sores or inflammation.
Does your baby turn to sounds as he did before being sick? Decreased responsiveness to sounds could suggest pressure or congestion in the ear, which can occur without infection due to teething.
Mouth pain (especially toward the back of the jaw) can also radiate to the ears, so the grabbing of the ears doesn’t mean that the problem is with the ears themselves. Some babies just rub their ears as a self-soothing behavior.
Personally, I always used my son’s behavior as an indicator of urgency. As long as he wasn’t clearly in pain or distress or acting lethargic, I let his body do what it does naturally. You say that your son slept well and is behaving like his normal self, other than drinking milk. In your case, my main concern would be hydration and nutrition… is he taking in any foods or liquids?
People are suggesting to make an appointment “today or tomorrow” – the point here is that you already have an appointment tomorrow – Thursday – and your question was whether it’s OK to wait one more day. If your child is taking in foods and liquids and wetting/soiling diapers normally, and if there are no signs of fever or acute pain, I think it would be OK to wait.