DS2's symptoms were vomiting, reflux and mucusy green poo. When I reintroduced milk into my (not his diet) he was fine on Day 1, a bit fussy on Day 2 but by Day 3 the vomiting, green poos and reflux were back.
I did two trials (at 4 and 6 months) and each time I had to increase the med dosage afterwards. Your LO is older though, so you might be OK. Paediatrician said any reaction would show by 3-7 days.