My 4 month old baby has an allergic reaction to milk formula and has switched to soy without problems. When should he have an allergy test?

No rush if soy OK. Was the reaction skin, respiratory or gastrointestinal (gi)? Was there blood in the feces? If symptoms were completely GI & associated with pallor or floppiness think enterocolitis. This non-ige mediated milk allergy reaction may persist for several years. Only valid test is oral challenge carefully performed by an experienced allergist. For skin & respiratory symptoms skin or blood test at 1 year.
12 months. Most milk allergies are outgrown-often by the age of 2. Blood tests can be done annually to measure specific allergy antibody to milk and when the level has dropped significantly an in-office oral challenge could be arranged. Would recommend the first blood test after 12 months of age.