Yes...weigh him. Do you have his birth weight? Kittens need to gain weight steadily, each day, and will have noticeable growth each week. Here are the averages, which are, of course, a very basic guideline...many kittens are smaller, and some are larger, and grow at various speeds. This is simply the norm:
At birth, weight should be approx 3-3.4 ounces.
At age 1 week weight should be approx. 4 ounces.
At age 2 weeks, weight should be approx. 7 ounces.
At age 3 weeks, weight should be approx. 10 ounces.
At age 4 weeks weight should be approx. 13 ounces.
At age 5 weeks, weight should be approximately 1 pound.
By the end of the 8th week, kittens should weigh approximately 2.2 pounds.
If he really isn't latching on, or getting the hang of nursing, chances are, he's not getting enough to eat...he may or may not get the hang of it, and chances are, there's a reason for this. You really may want to consider providing supplemental feedings to him, just to make sure he's getting enough in his tummy, so that he'll continue to thrive. Purchase a syringe with a plunger, a dropper, and a kitten bottle), and some KMR formula. Having all 3 on hand to experiment with makes it easier if one method doesn't work as well. Allow him to try to latch on and nurse from momma, and if he's crying, and not getting the hang of it, he's hungry, and needs your help.
First of all, weigh him NOW. Then, allow him to attempt nursing a couple more times, and then weigh him in the morning again, if he still appears healthy, and if he actually had something to eat. Check and see if the weight has either remained the same, or if he's actually gained a bit. If he doesn't succeed right away in nursing from momma today, run right out and get your supplies...he won't last long if he isn't getting adequate nourishment. You may want to attempt to feed him separately tonight, if he truly isn't eating from momma.
Please visit www.kitten-rescue.com
for specific feeding methods, and how to hand-raise a kitten. If runt-kitty isn't really thriving or getting the hang of nursing, I suggest you intervene, and start to feed him by hand. Please read up on the proper techniques, and how to make it safe, and easier for you and kitten. Good luck!