Questions about Pattern Sizes and Reducing to smaller

I have found two sweaters my son likes and now have to figure out how to resize them so they will fit him. He is a very petite and thin young man of 32 years. His size is the result of kidney disease, which he was born with and eventually required a kidney transplant that was successful. If a sweater gives the gauge of 17 stitches and 20 rows to 4" over k1 p3 rib on size 10 needles, could I change to needles 7 and 8 to reduce the size somewhat if I can get the gauge right? Also he requested a sport weight yarn, nothing too heavy since our Fl winters aren’t always cold. Winter here is a mix of cold, and warm days, cold can go into freezing temps but instead of staying there for days, the temps will start rising when the sun comes up. I have seen the temperature go from 29 degrees one morning to a more normal temp of 58 degrees later in the afternoon.
I will be waiting for your suggestions and advice.

It depends on the yarn you use. As the gauge in the pattern is given on size 10 needles, it’s probably a bulky yarn, so if you use that with size 7 and 8 you’ll a smaller sweater, but one that’s very stiff and heavy. However, if you use a lighter yarn, such as worsted, with the 7/8s and follow the pattern, it will reduce the size also and be more appropriate for the needle size. What you have to do is knit up the yarn with those needles and determine your sts/inch. Then multiply by the measurement you need to get the number of sts for both front and back of the sweater. See if one of the sizes is close to that and follow the instructions for it, adjusting for length. Sizing for men came up in another thread - they prefer looser sweaters than women, so allow about 4" extra in your measurement, which would mean you want to aim for about 36".

I will certianly try that