Why would my 8 month old be not putting on weight?

[2point4kids]

Had DS weighed a month ago after about 3 months of not taking him.
He'd put on weight over the 3 months, but less than he needed to to stay on his 50th percent line. He was on the 25th line.

I was told to encourage more solids and to bring him back in a month.

Went back today and he weighs exactly the same as he did last time! Now on 15th line.

HV said try not to worry because he's not losing weight, just not gaining.
I'm to go back in 2 weeks now to check he doesnt lose weight now.

He has 4 bottles a day plus 3 meals. He doesnt have a large appetite but drinks most of his bottles and eats most of his food!

Less calories, more exercise.
If you want to encourage weight gain you need to up the milk intake or give fattier food.

No, he doesnt crawl yet.
He can only roll one way so isnt rolling around either.
He can sit up though and mostly sits and plays with toys or is carried by me.