Some other ideas since this change has only occurred in the last week: Look for other recent changes...
Have you taken his temp, is it elevated at all above normal? There could be an infection. If he is crusing and crawling and walking he could have ingested some bacteria. Since he has been on the same formula, I would be inclined to think it is not the milk in the formula that is causing the sudden problem in the last week, unless you switched makers or the product switched (or added)ingredients to the milk.
Strep bacteria and Pneumococcal bacteria both cause vomiting.Pneumococcal can also have a cough with it. Cryptospordium and Giardia protozoa, which can be in drinking water or food, can cause vomiting and diarrhea.
Are you adding supplements. Some of the supplements have very high amounts of Iron, plus almost all formula has Iron, plus most baby food is enriched with Iron. Iron overdose is common in young children because Iron builds up in body stores. They need such small amounts. I have read some experts say that babies are born with all the Iron they need for the first 6-12 months of life. Iron poisoning causes vomiting among other symptoms, and so does Iron deficiency. But with all the Iron supplementing in products these days that is rare unless the child has some type of bleeding disorder or parasite.
Other mineral deficiencies can cause vomiting. Magnesium can help with vomiting in the case of GERD.
Prescription medication can cause vomiting and diarrhea.
Could there be additives in the baby food causing the allergy? Some allergens are gluten, MSG, soy, and wheat, as well as dairy.
Excessive vomiting and diarrhea can cause potassium deficiency...and dehydration. Potassium regulates water storage in the body.(magnesium helps regulate it in the cell). Foods you can feed him to keep potassium up are bananas, apricots, beef, chicken, salmon, and lima beans. Organics have higher mineral levels than regularly grown foods. Small amounts of magnesium oil rubbed on the skin can transdemally give magnesium. check with your doctor first.
I suggest keeping a diary. I did this with all my kids, even when they were well. ..so that I could keep track of when they ate and when their naps were due. It is especially good to keep track of anything ingested while an ilness is occuring, and any other symptoms, even if you think they are not related. For example you already mentioned congestion. Sleeping longer than normal, Drinking more than normal, eating less than normal, not playing like normal, changes to skin color, rashes of any kind, bluesih color to lips or under eyes, holding the body in different positions than normal, etc.....anything out of the ordinary.
My firstborn was allergic to milk, if it even touched her skin she would get a rash. We never fed her milk based formula, we fed soy. I would highly advise against soy, it is estrogenic, equivalent to giving 5 birth control pills a day. I wish I would have done more research on it back then. The research is all over the web. She now has arthritis and thyroid/adrenal issues,(at 12) and reports online link children with thyroid and arthritis to soy infant feeding. She is now on rice milk.
Anyway, I hope you find what is causing your baby's distress.
Best wishes,
Gail