C.
Sounds to me that he's developed the habit of needing to suck in order to sleep. Sucking has become a crutch for him for being able to sleep restfully. Sounds like he is kinda startling himself awake when he realizes he's not sucking anymore. What I would do is work to change that habit, which means not nursing to fall asleep every time so that he is able to learn to sleep without sucking. What really helps with it is doing a pattern of eating right when he wakes up in the morning, then wake/play time, then nap (not nursing to nap, so a nap about 1.5 hours after he begins eating if he eats about every 3 hours and then when he eats about every four hours going down for a nap after 2 hours so he's napping the second half of the eating increment...that way he's not eating to be able to sleep and he's getting a good time chunk to sleep without waking up short because of hunger), then feed again when he wakes and do the pattern again throughout the rest of the day. The only time he goes to sleep just after eating would be bedtime and then that also helps to tell him it is time for the long night time rest as opposed to the shorter daytime nap. If you give this like a week, even within a few days, I'm sure he'll be sleeping a lot better. For that first week, you may need to wake him to eat so he doesn't mess up the other naps and wind up needing to eat when he's ready for a nap, but within a week, he will very likely be keeping himself to the routine, as it really helps them in a lot of ways, particularly with a healthy sleeping pattern. What I did approximately
6am - eat
7:30 - nap
9am - eat
10:30 - nap
12 - eat
1:30 - nap
3pm - eat
4:30 - nap
6pm - eat
7:30 - nap
9pm - eat and then bed for the night
Or if he eats about every four hours, something like this:
6am - eat
8am - nap
10 - eat
12 - nap
2pm - eat
4pm - nap
6pm - eat
8pm - nap
10pm - eat & then to bed
Then of course as they get older, you adjust it as he needs, but the routine really helps with great sleep without a crutch.
I also use white noise machines to block out noises that may startle them as well.
Best wishes to you!!