It may take your eyes some time to get used to them, if you still struggle, see the optometrist/eye doctor again, and try a different lens material/brand (there are loads of brands!).
Astigmatism is a very common, and normal thing to have, and may be corrected by using specs, contact lenses, and laser eye surgery provided you are within the bounds of it.
Most people have some level of astigmatism. It does not however cause any obvious differences in the way your eyes 'look', if you're not sure, do book in to have it looked at!
Firstly, please have an eye test to make sure that's the only reason your eyes feel strained. Secondly, try and have a break from your computer, follow the 20-20-20 rule which is: every 20 mins, look away from your screen for 20 seconds at an object 20 feet away.
I understand your concern, what you have said is commonly called astigmatism, which is a very normal thing to have. Because of your son's age, make sure he wears glasses all the time (or as advised by the prescribing doctor). Also, make sure he spends more time in the outdoors as well, not just indoors looking at screens.
It could be the fact that you may need to get weaker glasses, get used to those, then slowly increase it to the full prescription. It is something we have to do quite often, so please don't worry, see your optometrist/ophthalmic specialist who prescribed you the specs.
It may be that the glasses were probably prescribed for that very reason, so that your eyes don't get crossed. Additionally, you may want to have another eye test to make sure that the muscles of your eyes are working as they should, and that this 'crossed eye' appearance isn't something new. Possibly also consider contact lenses.
During an eye test, the health of the eyes are generally checked out as well, so if there was an underlying issue, we would be able to pick it up.
If you're not sure, get a second opinion.
Also, with regards to the glasses, it's very hard to predict when the vision will stabilise, please be assured that vision and health can be 2 different things, and even someone with very healthy eyes may need stronger glasses every year.
Also, with regards to foods, go for antioxidant rich foods, so lots of greens, berries, and lots of carrots (its not a myth!).