If your baby is growing and doing well otherwise, then you are probably not far off here A few extra feeds might be all it takes to get the weight gains again and set you back on track.

I have to say, your doctor has given you some pretty crappy advice in regards to breastfeeding
How many times are you feeding in 24 hours generally? every 1.5 hours sounds very normal to me. How many wet & dirty nappies is your son doing? That is the best indication of how much milk is going in.
There's no reason to stop him feeding after any set length of time. One thing you might want to try is to take him off after 20 minutes or so, burp him, and put him on the other side. Then repeat and back to the first side again, till he's done.
It is very, very common for babies to 'regress' on sleep at this age. It has nothing to do with their milk intake and does not mean they need formula or solids. They tend to go through a developmental stage where they become more aware and and active and may be fussy at the breast. Taking them to a darkened, quiet room to feed may help. You might find they feed more often over night as they are not getting as much during the day (since they are so distracted). It's quite normal and just a phase that will pass.

Unfortunately many people interpret this behaviour as meaning they suddenly don't have enough milk. But that's not the way breastfeeding works. If you feed, you will make milk. Worrying about it also will not affect your supply.

The slow weight gain is always a worry for parents. Has he grown otherwise? More active perhaps? I would suggest first, calling a lactaction consultant for expert advice, and or the ABA (they also may be able to recommend an LC in your area).
Unfortunately many GPs, nurses etc know very little about breastfeeding and will often give quite bad advice. I think it's also important to know that solids at this age will not ensure the same nutrition and calorific intake for your baby as milk will, particularly because the sorts of things we tend to give young babies are very low calorie. More milk is a much better option.

I think all you need here is a little fine tuning and some help from someone with a some expertise, and you'll both be right You're doing well to follow your baby's cues and your instincts.
Good luck!