Hello Tania,
this shouldnt be bother you much, wait till your baby turns 6 months and he must be double his weight by then and that is considered to be normal. just make sure that baby is playful, active , taking feed well and urinating well(6-8times/day)
if you have further doubts on this query then please book an appointment for virtual consultation with pediatrician at https://bit.ly/3CfJAug5