Hi,
Babies under one year old almost never wean themselves, they may go on nursing strike or just simply prefer the faster flow of milk from the bottle.
If you supplement with formula it is quite normal that your supply will suffer because of this. The rule is simple, you are sending your body a message that less milk needs to be produced as your son is getting fewer breastfeeds. Demand=supply.
I understand that you might have had problems with expressing when you are away from him because baby is much better at expressing your milk than a breast pump. And if you have started working, you are probably under stress, are more tired and so on. These all are factors that influence milk supply.
It really depends on you, how determined you are to keep him only on breastmilk vs formula. You might work on increasing your supply by offering him to nurse more often (the best way), drinking a lot of water, taking Fenugreek, this is a herb known for increasing milk supply.
He is almost 6 months old, soon you will be able to offer him some solids, so if you are concerned that he needs more food, you will have some solids to offer him. If you want to stick to breastfeeding, try to decrease use of bottles and increase the amount of breastfeedings. He probably prefers bottles now as he realized he needs no effort to get his milk out of them. Also, if you already give him one, offer one with a slow flow nipple, that way he will have to work harder to eat.
I always strongly recommend nursing if possible. If you can't commit yourself to 100% breastfeeding, try to keep at least some of his nursings, breast milk is so much better for him than formula. Keep in mind that the more formula you use the lesser milk supply you will have.
If you want more info on increasing your supply let me know.
