About baby's weight gain!!

Hello everyone, can u please tell how much does ur little one's are gaining their weight? N wat is considered normal average weight gain.. please reply if possible i wanna know... my lil boy was born 3.5kgs at born n now at 6weeks gains only 3.8kgs n doctor says its less weight so i am very much concerned about it.. can u all suggest what should i do to increase his weight? He is only breastfed. Please let me know if anyone can help n also how ur lil one's are gaining weight. Thank You.