If I were you, I'd stay on the Alimentum for a few more weeks to make sure, because it may take a few weeks for the effects of the previous formula/breast milk to go away. If you do want to go ahead and find a dairy-free formula, here are some I found with a quick search:
Neocate and EleCare are completely free of allergenic proteins. Ross makes a soy-based infant formula called Similac Isomil and also Similac Isomil Advance Soy Formula with Iron. Check the baby aisle in the grocery store, and ask your son's pediatrician for ideas, too. Ask a pharmacist for samples of Neocate or EleCare or any of the other formulas. Most pharmacies can order these formulas for you if a grocery store doesn't carry them on the shelf. Or you can buy them online.
Here are some web links:
http://abbottnutrition.com/products/products.aspx?pid=236 - Similac Isomil
http://abbottnutrition.com/products/products.aspx?pid=47 - Similac Isomil Advance
http://abbottnutrition.com/products/products.aspx?pid=11 - EleCare
http://www.shsna.com/pages/dha_ara.htm - Neocate Infant with DHA and ARA
http://www.shsna.com/pages/neocate_inf_formula.htm - Neocate Infant
I hope this helps - take care,
Gluten & Sulfite Sensitivity, Multiple Food & Inhalant & Medication Allergies, Asthma, Gut issues (dysmotility, non-specific inflammation), UCTD (Lupus?), Osteoporosis, Anemia, Very Low Lymphocyte Counts (T-Cells & B-Cells), malabsorption/malnutrition, etc.
Meds: Pulmicort, Injectable Vitamin B12, Herbs, Nutritional Supplements, Essential Oils, Homeopathy.