What centile is your son? Is he consistently dropping lower and lower, or are there dips and rises on the centile charts? How is his eating and drinking? Does he wet diapers, and have at least 1 soiled diaper a day?
All I have to say is that don't take your doctor's words, etc., as golden. We had 'issues' with our LO and her weight gain--she was only 13lb 14 oz or so @ 6 mo, and then her weight stalled from 6-9 months (only 6oz gained then--so she was 14lb, 4 oz @ 9 months, and dropped off the charts.). Now, at 14 mo, she is between 17-18 lbs and doing just fine. Walking, saying a few words, gaining motor skills, starting to socialize with other babies and children especially.
Our initial GP doc referred us to a ped and mentioned possibly putting her on some kind of pill to increase her appetite. Because heaven forbid the idea that SOME babies are meant to be on the smaller end.
I would ask for a ped referral. Our new ped (didn't like the one the doc referred us to either), gave us a second round of blood work, urine samples the first ped never ordered, and stool samples, and eventually, all came out fine. The first ped alarmed us about cystic fibrosis and yet never ordered a test for it, which boggles my mind.
I would say that it is normal to be concerned with your son especially as there is an indication something might be wrong, but don't be hard on yourself and just do what you have to do to get the best care for your son. Sometimes it may not be something major that has to be rectified in order to help him thrive. But if it is something major, you just take it as it comes. Sometimes we need to advocate for ourselves (as we had done--going to the children's hospital and getting a second ped referral). And we quite like her, as she is not concerned about our LO at all now, because Valla had continued to gain and is back on her curve, developing normally, etc.
I was TERRIFIED that my child might have Cystic Fibrosis, and why? Because like any other parent, I want a perfectly healthy baby. But then I came to the realization that nothing would really change if she were diagnosed with it. Her care may vary, sure, but so be it. We won't love her or treat her any less, she is perfect, no matter what may be 'wrong.' I placed my faith and my daughter's health in God's hands, (to me, as it should be). And I've made peace with it.
Take good care and let us know how the little guy is doing.