Formulas all taste different (I've tasted most of them), and baby tummies react differently to different kinds of formula (similac "gentle proteins" for example, made my son *violently* ill... the formula is already partially digested, so it also goes bad VERY quickly... inside of 30 minutes... powdered formula gave him terrible gas and constipation... through trial and error, we found the only formula that worked for my son was enfamil lipil premixed - it has a silky consistency that is impossible to achieve with powder. For other babies, other formulas work). There is a LOT of trial and error with formula. We didn't have a choice, because at 9mo I lost my milk, so we just had to keep spending and trying to find something that worked.
Your son doesn't need solids yet, and in fact, many babies can't digest them until around 1 year old, in any event. Solids for most babies are like eating metamucil or cardboard for us. No nutritional value. When he starts going after your food (because it will start to smell good to them when their system *Starts* to be able to digest it), go ahead and start giving him some of what you're eating. Don't stress about it prior, or even then.
He may only be drinking teensy amounts of milk while you're working, but he may well be drinking gallons when he's nursing. When my schedule would change it would take my son about a week to adjust so that he was only taking 1 bottle while I was gone, tops, and would just nurse like crazy when I got home. If he's hungry, he won't starve himself, but babies are very quick to catch onto how things work. I also couldn't pump. Tried. A lot. Didn't work. But my son was nursing over 20oz of milk each time he nursed. Sigh. So if he's eating fine, but pumping is a nightmare, quit.
I got my period about 3 weeks after I quit bleeding from the birth. I had no problem whatsoever nursing until I got sick at 9mo (massive systemic infection). Just because your cycle has returned doesn't mean it's going to impact your supply / ability to nurse.
So just keep nursing as long as you're able and try out different kinds of formula, and BREATHE. Relax. You two will be fine.