visiting Grandma & Grandpa in NYS - need carseat help

NYBrideInWI

Hi Ladies, Hoping you can help me with what you use for carseats (I'm planning ahead and researching). Is a highback "5-point harness" booster legal in New York for a 3 year old let's figure he'll weight 38-40 lbs? I can't quite figure out what the law is saying.

LAW: "Must be in child restraint"

NYS: 3 and younger unless they weigh more than 40 pounds and are seated where there is no available lap/shoulder belt; 4 through 7 years unless they are seated where there is no available lap/shoulder belt 

source http://www.iihs.org/laws/ChildRestraint.aspx 

We visit my parents in Rochester several times a year and its been SO helpful for them to have a carseat for DS (#1) so I don't have to take one on the plane. This winter we have baby #2 (DD) coming and need a second seat for grandparents' house. I've noticed the inexpensive (<$100) convertible seats max out at 40lbs so I don't want to buy one if DS will be over the weight limit, but he seems young to be in a booster seat. He's about 36-37 lbs already.

This is the seat I'm thinking about buying (sorry can't make it clicky).

http://www.amazon.com/Safety-1st-Summit-Deluxe-Booster/dp/B001FB5BEI/ref=sr_1_8?s=baby-products&ie=UTF8&qid=1316400943&sr=1-8 

Thanks!