We have the Joie R129 Every Stage for both our boys (3yrs and 9 months) and would recommend. The only thing that annoys me is the seat belt gets in the way a bit when in rear facing mode but its a great budget seat that can be used as a booster seat once they've outgrown the rear facing mode.
Have you got a picture of him in his current seat? Just wondering if there's some adjustments you can make to the fit. As Reena said, spin seat isn't going to last you long, it will force you to turn him forward facing before he's old enough as he's larger than the average baby.
If your child is big at 5 months so 50th centile above I wouldn't recommend a spin seat or all stages. They only go to 105cm/18kg and baby will outgrow before they reach a safe booster height. Spin seats are also very bulky. All stages seats also aren't always the best for fit as they try to do too many things do they don't do one of the things well. The best thing would be to see a specialist retailer near you who sell the ERF 25/36kg seats and get advice from them and try the seats in your car.