I have no experience with reflux (although ds 8 months, has just been prescribed Gaviscon to see if reflux is causing his grumpiness) I doubt it as he sleeps so well at night but I thought it was worth a try.
Anyway, just wanted to sympathise, i know how hard grumpy babies can be. You sound like a completely devoted mummy and I think it's completely normal to worry about your babies health.
Of course, you want him to be healthy and happy. It is so so miserable when you seem to be the only one who is struggling to keep it together. My friend's seem to breeze through motherhood and their babies seem so bloody content. I always doubt myself and wonder what the hell I'm doing wrong! Thankfully, I've had many kind words from people on mumsnet in similar situations to me which does make it easier.
I used to really worry about ds' eating, he used to gag on the tiniest lumps and vomit as soon as he gagged. It is extremely hard to get him to eat solids as he is constantly trying to get out of the highchair, crying, wriggling, arching his back. I have found it a challenge but he is getting better. He does really well with finger foods, although he still gags he doesn't do it as often and is rarely sick.
I would deffinitely go back to your doctors and voice your concerns. I know how tempting it is to stay at home and make out everythings fine but really, you need to tell your doctor how you feel. You know your baby and if you are not satisfied then tell them. I actually just changed doctors a couple of weeks ago as my last one was crap with babies and just didn't care. The new one is lovely and I feel much more comfortable talking to her. Perhaps you could find a new doctor or health visitor to talk to.
The bottom line is that there is plenty of time to increase solids. He is getting plenty of milk so he will be fine.
I know you feel useless and really aren't enjoying motherhood right now. There are lots of people who feel this for the first year or so. Things can be very hard and look very bleak. I felt like a bag of crap the other week and couldn't stop sobbing at what a shit mother I was and how all my friends can cope when i'm so crap. It's hard and I sympathise with you. A lot of what you are saying reminded me of exactly how I feel sometimes. Thank God I get to sleep at night. You must be a bloody saint!You are doing great in a very difficult and worrying situation.
Please don't see it as your failings. Things will get better. You will enjoy your baby. I keep counting down to ds' first birthday (not that things will miracullously get better then) but I feel it is my aim at the moment and then I can hopefully assess the year and see how far we've come. Only 16 weeks to go! You are doing fine. Sending you hugs though! x