From what I understand the Evenflo Tributes were the latest seats given out. Is that what you have? Even though he is at the second to top shoulder slots he may be there for quite sometime like Sarahs wee boy Floyd. Remember babies can grow more in the legs than the torso and vice-versa.
We have a Cosco Scenera (similar kind of convertible seat with same RFing weight limit (15.8kgs) and when we put her in it at 5mths (about 68cms and 8kgs) she was on the 2nd to top shoulder slot and at 13mths she is still there. (about 78cm and 11.5kgs)
Depending on how long you want to keep your baby RFing you may want to look at saving for a seat with a higher RFing limit.
The extended RFing thread is
here It's in the News Forum. Lots of info on extended RFing seats