I give my son Motrin mostly at nap and bedtime, so usually only twice per day, but you can give it three times if you need to (every 8 hrs.). The dose is based on weight. If you buy the infants motrin it does tell you how much to give according to the weight.
My son usually eats poorly just for a few days when the teeth are actually cutting through the skin. The teeth do take a long time to break through (sometimes weeks), but he usually eats o.k. except for those really bad days.
I hope Brian feels better soon. My little guy is cutting a tooth today and has been miserable, so I know what you’re going through. It’s a bummer