Pretty much what Kreeli said
At this age (early toddlerhood), there are a lot of frustrations for children. They're mobile, but they can't reach everything they want to examine. They can communicate a little bit with you, but can't tell you exactly what they're thinking. And they don't always know exactly what
it is that they want or need... just that they want or need something
. No wonder we sometimes want to cry right along with them!
(Digression on communication: I'm an advocate for signing with your baby/toddler... see http://www.sign2me.com
... it's never too late to begin, and rather than slowing language development, it actually reinforces verbal language and eases communication frustration.)
Something else to consider: as a nanny, I've found that often "cranky" or "fussy" children are not getting sufficient sleep or have unhealthy sleep patterns. This may or may not be the case for you, but it could be a factor. I'd be interested in learning more about his sleep patterns and habits.
As you say, it definitely could be that he's so used to being entertained by his grandparents, and not used to finding a way to cope with frustrations that pop up. There is such thing as a healthy amount of frustration--it's what inspires children to acquire new skills and learn to solve problems. For instance, if a parent/caregiver always
helps a child figure out how to work a certain toy when it gets frustrating, the child doesn't have any impetus to figure it out for himself.
Is Aiden mostly crying when he's out of eyeshot of you? He could be going through some separation anxiety. Play yards are great for times when you need to mop the floor and don't want him to get into the dirty water, but he wants to be within eyeballing distance of you. Some parents are opposed to any
use of a play yard, but I think it's a matter of practicality. Certainly you don't want to overuse it, but a few minutes once or twice a day is not negligent.
Kreeli gives great suggestions on how to transition your child into his own play explorations. And I definitely agree with her that it's important for youngsters to learn to play independently sometimes. Aiden is a very important responsibility in your life, but not the ONLY responsibility you have. This is hard for a child to understand, having little concept of time or responsibility, but it doesn't change the reality of the situation. Mommies and daddies have things they have to take care of sometimes--and remember that you have rights too.
Perhaps you could set a timer for 5 minutes (at this age, I'd say that's quite a long time!) and say, "When the bell goes DING!, Mommy will play with you for a little bit." Then, of course, you must follow through when the bell rings, even if you're not finished with what you're doing. As Aiden becomes used to this and trusts that you will always be true to your word, you can increase the time by a minute or two, until he is able to play by himself for 20 to 30 minutes at a time a few times a day.
Are there ways that you can involve Aiden in what you're doing? When I fold laundry, for example, I fold it on top of the table where the little girl in my care can't pull down the stacks I've folded. Then, I give her a stack of cloth diapers and talk to her about folding the clothes. She waves them around and puts them in piles and generally imitates me in her 13-month-old way. Too cute! When I'm wiping down the countertop after a meal, I give her a wet washcloth and she "wipes" her highchair tray. Maybe when you're mopping the floor, you could set him in his high chair with a sandpail of water (half an inch should suffice!)... he can spash and slosh it around, and it doesn't make that much of a difference since you've got the floor wet anyway. Just a thought.
Also, it helps to remember that each child has a different temperament and personality. It's hard not to take the fussing personally sometimes, but your child may just have a lower threshold for frustration than, say, the 8-month-old girl you mention... especially considering that they're at very different developmental stages.
Keep writing, especially if you need to vent some Mommy Steam
Let us know how things go!