If your paediatrician says your baby is too thin then she will advise you what course to take. If friends/family say the baby is too thin AND you agree with them then again, the paediatrician is the only person to take advice from. This really is not a question for QL other than to ask where to get the correct advice. If this is your first baby it is difficult to know the right thing to do and who to listen to. Always ask these serious questions of a professional. If you are not completely satisfied with this advice, GET A SECOND OPINION. If your baby is listless and not appearing to thrive then it is also necessary to seek paediatric advice.
A five month old baby should be having regular medical check-ups at this age, regardless of the state of its health. Find a doctor you are happy with and be guided as to how often you need to make appointments for weight monitoring.
Sounds like you are doing a fine job. Just get reassurance.