Maybe your mother in law has been carrying and feed him since born, thats why he feel more secure with her especially now your time with him is lessen.
What I do everyday, during lunch time, call back to video call baby and show her my face. Then I go back hm, I insist to breastfeed her and bathe her.
Bathing baby in the evening I think is better, it will soothe baby after a long day in the warm bath, I can also sing and talk to her in the bath (bonding) then after that massage her abit, stroke her and teach her call mommy! lol... she will sleep very well after bath also.
But they say during their 5th to 6th month, babies will develop to cling to mommy alot, so far my baby doesn't cling to me...