Baby weight not gaining and not feeding.

My baby boy's birth weight is 3.3 kgs. At 60 days his weight is 5 kgs. By today his age is 3 months 14 days (104 days) his weight is 5.6 kgs. His height is 60 cms and ofc 39 cms. Whether he is gaining weight in a well manner or slow. he is only breast feeding. He is not feeding well like before. When we try to feed and try to insert nipple feeding he starts crying. Otherwise he is active no crying. He is only taking milk 4 hours once and that time is also a small amount. We are using neopetine syrup daily 16 drops since from birth. Also his tongue is turning black with using of multivitamin syrup and reducing with the stop of that syrup. My doubt is why my baby only increased 600 gms in 45 days. Please help us he is not taking feed