You've been given some great advice already but I'll share some of our story. Sorry if it's all over the place, it's 3am and I'm up with a restless (well grown) toddler

My DD was a very slow gainer and such a tiny little thing. She didn't wear 000 clothes until she was 3.5 months old! She was 5.5kg at 4 months and I tried Farax at about 4.5 months but only once and didn't attempt solids again (pureed fruit & vegies - that Farax is wrong!) until DD was almost 6 months. She was so much more ready for solids then. Some kids do well starting solids earlier (I'm just talking from experience of friends babies) but there really is no reason to start early.

As DD was such a slow gainer (we had quite a lot of BF issues) I was quite obsessed with weighing her but I always did it when she was naked. Definitely do that and try to use the same scales. A drop in session at the chemist is perfect. I managed to borrow some proper infant scales from a friend with a premmie at one stage so I could weigh DD before and after each breast to check how much she was getting from each side. That was so interesting and really uplifting. My LC had done this with her in the early weeks too. I highly recommend doing it if you can as it can really help ease your mind.

DD had top ups after every feed until about 4 months (I think) and we continued to offer them randomly after that point, when she appeared to need or want more milk, to help with her weight gain. That certainly helped a lot.

DD continued to grow and thrive and hung around the 50th percentile (I'm sure she was well below that for her first 4 months) but certainly didn't need solids at 4 months, milk was perfect for her.

Hang in there and trust your instincts. Your LO seems to be doing just fine.