Wow, first off I really commend you for doing what you are doing... you are creating a future for you and your son. What a strong woman you must be!
thankfully, your Mom is a very familiar person in his life and can care for him. I take it that she is nurturing and a good caring Grandma...?
Next, well Is there any way possible, that he can perhaps visit you where you are?
Or, via computer... perhaps you can get a computer video camera thing (those 'eyeball' cameras)... and then that way, you two can "see" each other?
At this age it's hard...because they are still so young... but not yet old enough or mature enough to fully understand where their Mommy is, and why. Thus, what your son is experiencing seems like he is a little depressed.... since he is withdrawing etc. And I'm sure he is feeling insecure about it all too...
Whatever happens, he should not get scolded or punished for his feelings and emotions, nor forced to be perfect or be 'strong' for such a young age. Perhaps... encouraging him to fully express his feelings might be a good outlet for him... boys NEED to learn to express themselves. So they don't get pent up and frustrated. It is a real harm to boys, when they are told things like "be strong... don't cry..." for example.
Next, does your son have any cousins he can play with at home? Perhaps, having some close relatives come over for play dates can help as well. What does your Mom do with him when he is lonely, or when he is acting depressed? THAT can make all the difference as well. What activities do they do together or what is their routine?
Perhaps, give your son an incentive... each week. (since kids at this age don't have a sense of 'time')... for example, "after Mommy calls you tonight... can you make me a picture of your truck? I would LOVE to have a picture you drew you are such a good artist." Or, take photos of where you are and what you do... and give it to him so that he can 'relate' to where you are and what you are doing. It might make him feel a little more secure when you are away....and helps him to form a idea of where you are instead of him just wondering about it. For example: photos of you studying... of you at your paramedic truck, of you at your internship site, of you in the cafeteria etc. Just things you do in your day to day responsibilities. That may be a better "visual" for him, so that he knows his Mommy is okay and misses him too.
I know it must be so hard....and you are doing something so important... and he is so young. I really give you credit for that. I hope some of these ideas helps... but if your son gets too depressed...then you and your Mom must watch out for that. Young children do NOT have 'coping skills' nor the instinctive ability to know how to behave under duress.
Or perhaps, ask your Pediatrician for ideas..
All the best to you and take care,
Susan