GER, GERD, reflux and congestion

I’ve been to a doctor for my baby’s congestion at least 4 times since he was 3 months old now he’s 6 months. They always say it’s cause he’s in daycare and it’s just an upper respiratory infection and it’s all in his nose. Recently he’s been refusing to drink. He doesn’t like many foods that we are trying to introduce. He was 20lbs a month ago and is in 4oz every 4 hrs. He’s still having a lot of wet diapers but he’s recently started vomiting or spitting up sometimes a bottle or 2. He will cough and sometimes just do it. I now suspect it’s GER or GERD or acid reflux. He does arch his back sometimes. He doesn’t cry often but he’s hard to feed despite his size. He gags a lot. When it comes up sometimes it has mucus. Can anyone share how their LO was tested for reflux and remedies or treatments? Idk I feel hopeful about this and I just want to find something to help my kid.