I am a Certified Ophthalmic Technician and have been involved in the eye care field for over 8 years.
First of all, it is great that you noticed this problem with you son and had it evaluated! It is very common for young children to have an eye turn in and glasses often help. However, it is very important for him to wear the glasses ALL THE TIME. (I have an 18-mo-old daughter who won't leave bows in her hair, so I completely understand that it may be a struggle. With an active toddler, the glasses may need to be adjusted often, possibly weekly, so they fit well.) Because you son's eyes need glasses, but haven't had them yet, his eyes are used to working hard to try to see (unsuccessfully). When he first wears the glasses, IT WILL BE BLURRY FOR HIM even with the correct prescription. Of course he won't want to wear them. But once his eyes relax (this could take several weeks) and let the glasses work for him, they should help. If he only wears the glasses intermittently, it will take his eyes longer to relax, or they may not at all. Many children eventually grow out of it, but not all.
It is critical for his eyes to be able to see clearly now because his brain is trying to process what he sees, and is, in a sense, "learn" to see. Unfortunately, there is a time limit to this learning. After around 9 years old, the brain can no longer "learn" to see. Glasses won't help with this. If the brain only sees blurry images from his left eye, it may decide not to use it and rely solely on his right eye.
I hope this made sense and I haven't scared you. It's just that I've seen so many children whose parents haven't follow the doctor's instructions and then it's too late. So bottom line, you are doing the right thing -- stay with it!
PS All that said, I am not a doctor. A second opinion is often worth the peace of mind.