our son has had more sinus stuff going on this year and we started with NeilMed's pediatric nasal rinse system. it hasn't been easy to get another thing into the routine but it does help. it turns out he was allergic to our new kitty too, so we had him treated for it.
food allergy pops into my mind. hidden food allergy can really take a toll on the body and sends the immune response into hyper-overdrive making you tired. you can get him tested through bloodwork or something called NAET which is the system we used to eliminate our son's dairy/seasonal allergy. Here is a collection of some resources <a target=_blank class=ftalternatingbarlinklarge href="http://www.cfnutrition4life.com/cfnu/articles.php?cat_id=2
">http://www.cfnutrition4life.co...articles.php?cat_id=2
</a>
in our experience and after reading, we found too much reliance on milk and liquids in the diet can curb appetite and make it difficult to get other high calorie/nutrient rich foods into them. I would try only offering milk/smoothies after a meal or as a major part of a snack. toddlers are tough and what is normal eating distractions for their age often freaks us out to no end as parents of cfers. i try to find his favorites and add veggies in the form of purees whereever I can (ie pureed peas/spinach to meatloaf or meatballs). also I use coconut milk and oils alot to add calories. ground nuts (if not allergic) and pureed beans can be hidden in alot stuff too and adds calories and protein which the body can use alot.
some supplements which reduce inflammation could be added and a daily probiotic might help with digestion. we use DHA, tumeric, GSH, extra C, taurine, and chelated magnesium to help. we have been happy with the results.