Brief Answer:
MRI is normal, for ALS an EMG is most crucial test.
Detailed Answer:
I read your question carefully and I also reviewed the images that you kindly provided.
The MRI doesn't reveal any abnormalities, at least not in the brain (there are some changes indicating sinusitis but are not related to the symptoms).
Keep in mind that a brain MRI does not make a diagnosis of ALS. It is done merely to exclude other potential causes, such as tumor or stroke (I can confidently say they are excluded by this exam). The same applies for neck MRI which is also routinely done when ALS is suspected. The only test which shows changes due to the disease is EMG (electromyography) with nerve conduction studies. So if not already done it certainly should.
If I think ALS to be possible considering the symptoms you describe and their progressive evolution....I am afraid yes, it is well possible. ALS typical manifestation is progressive motor weakness involving gradually different regions, mainly the limbs and the muscles responsible for swallowing. So diagnosis would be consistent with symptoms. However the EMG must absolutely be done, in particular given his young age all possibilities should be excluded.
I remain at your disposal for other questions.