There are a couple of issues here.
First of all, it's alarming how big a small amount of milk looks, spilled out. If you take a couple of spoonsful of milk and spill them on your table, you can see what I mean. A full feed is quite a few ounces, and when they bring up a full feed, everyone is pretty much soaked to the waist, in my experience. (At any rate, if your DS was throwing up full feeds, he wouldn't be gaining weight, obviously.) Lots of babies just take too much milk and throw up the excess - my DS2 was like that. This is normal, nothing to worry about, and really just a laundry problem.
If it is reflux, as other posters have suggested, I don't think your DS would be gaining well, and he would be unhappy when he threw up, from what I know. How does he feel about the puking?
Next, straining to poo is normal for a breastfed baby, as the poos are so soft. It's annoying and probably unpleasant for them, but this isn't colic. Colic is lots of screaming, for no apparent reason, for several hours per day.
Giving a bottle might change things, but not necessarily for the better. It sounds like breastfeeding is working well. Seeing a breastfeeding counsellor to discuss all this, and check latch, wouldn't be a bad idea (no harm), but it really does sound like everything is fine and normal.