Normal weight and height vary widely. Do not expect even weight gain throughout. Some babies gain more weight during the first few months. You child also has gained more than one kilo during one month which is above average. You have not mentioned the birth weight
. I do not think there is any problem with his weight. Checking length accurately in small babies can be difficult at times. Again you have not mentioned the length at birth. If it was near the lower limit of normal I do not think there is any problem. Just take the child for regular follow up. I hope his development is normal.