It could be nipple confusion because milk flow is quite fast from bottle vs breast where baby has to put in suckling effort. So baby goes in for easy and faster milk.
But don't worry. Baby will start breastfeeding gradually with your effort and dedication.
At 1.5 month, your body is still struggling to make milk. As days pass by the flow of breast milk will be well established and baby will enjoy breastfeeding. So inorder to maintain the milk flow you try to empty the breasts by expressing whenever baby is refusing to feed directly at breast or even after he finishes his feed. Maintain a balanced diet. If the milk is less, you may try Lactare capsules (2 capsules three times a day for 10 days) to increase milk supply.
Give lot of skin-to-skin contact. This works wonders for your baby to start recognising you by your smell and get back to breast feed. You have to remove baby's clothes with just the nappy on and put him against your skin and cover yourselves both with a blanket.
Always offer to breastfeed first. If baby refuses you may give him initial 1-2 ounces of bottle feed to pacify him and then put him to breast. Maintain hygiene of the bottle nipples by proper sterilization. You may try a slow-flow nipple (like Medela Calma) to discourage baby to bottle feed.
Stay relaxed and positive. You will be able to breastfeed exclusively very soon. Good luck.
Health Tips
For learning the proper techniques of breastfeeding, latching etc you can meet a lactation counsellor