3 year old - Poor weight gain

Hi! So my 3-year-old has been struggling to gain weight since forever. He stays in the same weight for very long periods of time. At 2-year-old he was 22lbs. He turned 3 in January, and he only gained 1 pound (IN A YEAR😱) and his height has been the same struggle. I thought he was eating well, but in that last doctor’s appointment It was confirmed he obviously wasn’t. He’s allergic to milk and eggs so his diet has always been a bit restricted. But overall I think or, at least, thought he was eating okay. Is anyone going through the same thing?? I feel like the worst mother; like I don’t feed him enough 🥴😪