Hi C.!
I remember asking the same question when my son was born (he is now 2 years old) and I am currently using a blanket sleeper for my (almost) 6 month old daughter!
First of all, I'd recommend using the sleeveless type of blanket sleeper, mainly because the advertisements for the Halo (sleeveless) brand say that there is no risk of the babies "re-breathing" their own air with a sleeveless blanket sleeper. Not even sure if that is true or not, but it scared me into using a sleeveless kind! (BTW, did you know that you can get a nice sleeveless blanket sleeper at Target for $8, as opposed to the Halo brand, which costs about $20?) That said:
My children's rooms are always a little chilly. Always. I hate being hot so the air conditioning is always on in the summer months (and yes, even recently, with the weather being so warm. And for some reason, in the winter, their rooms are still chilly, even with the heat pumping through their rooms!
Also keep in mind that a blanket sleeper takes the place of a blanket.
What I do for my daughter is this: Aside from the fact that her room is cool, if the rest of the house is kind of warmish (in the warmer weather), I will put her in a light pair of pajamas (no onesie underneath), and I will put the blanket sleeper over that. This way, I know that the next morning when she wakes up, I can take the blanket sleeper off of her, and she will be comfy in her light pj's, until I am ready to put her clothes on. If the rest of the house is going to be coolish (like when the nights are really cool, even when the rest of the days seem warm!), I will put her in a warmer pair of pj's with the blanket sleeper over that.
Currently, I am using a cotton blanket sleeper. However, when it starts getting really cold outside (as it will be soon), and I know her room is going to be pretty darn chilly at night (know matter how much we crank up the heat!), I will be using a warmer, fleece blanket sleeper over her pj's.
I'm not sure how long this warm weather will be lasting, but if the room your son is sleeping in is really warm at night, feel free to just put him in a onesie. I know he is just over 2 months old, but what someone else said is true, about dressing the baby how YOU would personally feel comfortable. So if you are wearing just a t-shirt to bed, your baby will most likely be comfy in that onesie underneath the blanket sleeper as well.
You also have the option of NOT using the blanket sleeper at all, but of putting a onesie on him, and then putting either a light or warm pair of pj's on top of that (depending on the room's temperature.) The only reason I don't like doing that is, I'm afraid my daughter will be too hot in the morning, when she would normally have the blanket taken off of her because she's not in bed anymore! But with your son being so little (and the fact that cooler temps will be upon us soon!), I'm sure he can be just as comfy in a onesie and pj's as he would be with the onesie or pj's and the blanket sleeper.
God, I remember going through the same stuff as you! My son was also born in August (August 2), so yah, I was right there along with you! I was constantly changing my mind of what I should put him in at night, because I was always second-guessing myself! I'm sure your son doesn't sleep too often yet because he is so young, but if you feel he is more irritable than usual and just doesn't seem comfortable, it may be his way of telling you he is too warm/too cold. It is all trial and error, but soon you will be able to trust your own instincts!
Good luck, and congrats on your new little guy!