I had to go and get my red book to see what weight my DS was at about 5.5weeks. My DS at 6 weeks was 5.5kg, so very similar to your bub... My DS is fully breastfed as well, what can I say - he loves to eat... LOL. He is now going on 16 weeks (4 months) and he weighs approx. 7.5kg - so he is a big bubba and everyone keeps telling me so (which I am a bit over)... We are just doing a really really really good job Keep up the good work!!!