B12 Deficiency , IBS and Weight loss need your Advice Gymbros , 22M ,50kg , 5'10.

Guys , I am 22M , 50kg , 5'10, skinny since last 4 Yeats I have suffering with stomach issues , recently found out that I have IBS. Due to which I am unable to eat any related to lactose and due to strict diet because of doctor I lost 5kg in last 2 months. Recently found out that I have B12 deficiency also. Can you guys please help which type of Supplement protein and B12 Supplement I can take which will improve my health and help build muscle and how can I improve my health and grow big.

Thankyou guys any suggestion is appreciated.