Car seat question?

Emma • I`m Emma. I`m 23 and happily married to the love of my life. We welcomed a baby boy 5/13/16 by emergency c-section!
Have any of you stopped using the carrier and base and went to a regular car seat. My boy will be 5 months tomorrow and he's huge 18+ lbs and 27+ in. He's already getting too big for his carrier.  
Also Is it safe to put him next to a window? I have a older stepson in a booster and it's a pain in the ass to have them next to each other cause I can't buckle the seatbelt with the booster well when the carrier is clicked in. 
Thanks ladies!