Hi K.,
I'm a medical writer with expertise in the field of allergy (actually, I should be working on a review article right now, instead of this message ! ;-) )
Your son is exhibiting classic symptoms of food intolerance/allergy. Food intolerance can appear at any age, but can appear during the new born period because of sensitization that has occurred in the womb. At this age, the only way to tell if it's an actual allergy (to milk protein) vs. an intolerance to milk protein or lactose, would be to get a blood test for specific IgE (positive if a true allergy, negative if it's an intolerance). Since changing his diet has been effective in relieving his symptoms, however, this is fairly diagnostic for some kind of hypersensitivity or intolerance to milk. It really doesn't matter which at this point, since the treatment, i.e., dairy avoidance, is the same.
In case this is a true cow's milk allergy, it's very important to practice complete avoidance, especially since all dairy products will contain both lactose and casein, which will be carried in your milk. All forms in any amount should be avoided. Fortunately, a large percentage of children with early dairy allergy will "out grow" the allergy by the elementary or teen years. However, if avoidance is not practiced, food allergy can progress to inhalant allergy (dust mites, pollens, etc.), and culminate in asthma in adolescence. You should resume nursing (definitely preferable!), but cut out all dairy. Even if the symptoms resurface, they should abate as all of the dairy proteins "clear out" of your milk over a day or two. As your son gets older, you can try reintroducing dairy products, but be on the lookout for signs of intolerance: gassiness, diarrhea, nausea, vomiting, and eczema.
Just so you know, about 90% of people with cow's milk allergy are also allergic to goat's and sheep's milk -- the proteins are very similar-- so you should avoid these too.
There is usually a genetic component to allergy, although like any genetic disease, it can arise in a family with no history. If you look closely at your family for other symptoms, you might find that there is an allergic tendency that hasn't been recognized. For example, if Uncle Al always avoided certain foods because they "just didn't agree with him," this could be because he was actually allergic or intolerant to them, without the clinical diagnosis. Skin disorders--especially eczema and atopic dermatitis--frequently have their roots in allergy but have not been well recognized until the past couple of decades.
Something you should keep in mind is that your son may not have been responding to small amounts of dairy protein in your breast milk, but was reacting to something in one of the other formulas you were using. If they were cow's milk formulas, he could well have a cow's milk allergy that is evoked by the large amounts of casein in the formula, but that would not be evoked by small amounts of dairy protein in your milk. Or, since corn is also a major allergen, if the formulas contained corn syrup or other corn products, that could be causing the problem, as could any number of other components. Have you tried exclusively breast feeding your son? There's no real reason to supplement, unless you're trying to wean him to formula for a return to work. Even if Dad want's to give him a bottle, he can use your pumped milk (just leave the house if necessary, so your son can't smell you!). It's a bit harder when/if you return to work, but not impossible if you get a good Medela double breast pump.
You can try reintroducing dairy after the first year, if you decide that this is the true culprit. If symptoms arise again, it would be worthwhile getting him tested to rule allergy in or out. If he is only lactose intolerant, he could still enjoy lactase-treated dairy products, or chew lactaid tablets before eating dairy products. If he has a true allergy, it's good to know from the standpoint of avoidance. It's also worth retesting periodically in elementary school and adolescence to see if the he grows out of this. We are starting to gain a better idea of why some people outgrow certain allergies on the basis of molecular analysis, but this is still highly experimental and the cost of testing is prohibitive. It's much easier to just do an occasional food challenge, as long as there's no risk of anaphylaxis.
I hope this helps!