When we bring my boy for jab @ 6 weeks, he was around 5.05kg. So @ 8 wks should be around 5.2-5.3kg. Now he's @ 9 weeks.. never really go weigh him recently. Hee.
Now he's also drinking 120ml of fm... been on this vol. for 2-3wks, still wondering if i should increase his feed cos sometimes he can be hungry in 2 hours time but sometimes he can't even finish his 120ml & even throw out milk once in awhile (which we suspect cos he didnt swallow dwn the milk as he was sleeping while drinking the milk).
Anyway i dont think yr boy is overweight, maybe bones heavy? And so long as his weight is increasing, should be ok i think. you continue to monitor & see how it does. If you are still worried than bring him to the doc.