I think we used ours from about 2 mo? We didn't go out much before that.
Does the seat of your stroller recline? Until he had decent head/neck control, we kept the seat of the stroller reclined somewhat. And, when out at nap time, we reclined it all the way flat, then hung a lightweight blanket over the canopy to make it a bit darker & less distracting. We didn't have him sitting straight up in the stroller until 4-5 mo, whenever it was that he started complaining about being reclined.
Do you offer him toys to play with in the stroller or car seat?
Somewhere around 4mo we also put an extra stroller (hand-me-down) in the living room, and propped him in it (sitting up almost straight) with a rolled blanket on each side. Then toys in his lap. That way he could "sit" in the living room at our level, see the tv, etc. Also good sitting practice, and an easy way to move him from one room to another.