if your GP isn't worried, then i wouldn't be too worried. fwiw, my DS has cracked 8kg at 4 months (birth weight of 3.99) & was over 10kg at 8 months but then slowed right down because he was crawling & then walking - it was like he stocked up on weight to cope with being an extremely active little person!

i BF my DS so don't know about FF but at 6 months old i wouldn't cut back on milk too much cos it is still meant to be their main source of nutrition for another 6 months.