Firstly, most medical images are actually being saved in a standardized format (DICOM, as other people mentioned) and I suppose that was the case for your son's records, too. However, the devil lies in the details, which is reflected in your experience.
While the images are saved as DICOMs, additional information (e.g. orientation of the image, patient data, etc.) can be saved to the DICOM meta-tags. There is some standardization as to which type of information to write into which meta-tag id, some manufacturers however have varying implementations of these standards. The result may be inconsistent meta-tag structure and naming. Example: Trying to determine the type of MRI sequence (= an imaging technique) including general physical parameters like TR and TE from the DICOM files. Every clinic may name the sequence differently. The parameters TR and TE may be in different DICOM tags, depending on the MRI manufacturer. For a correct assessment of any foreign MRI image, the physician should know the parameters (they usually have no idea).
Secondly, regarding MRIs specifically, every clinic has a different so-called protocol for different diseases. A protocol consists of multiple sequences. Example: For Brain Tumors, clinic A might want to take an MRI with the sequences X and Y. Clinic B may additionally want to use sequence Z. Furthermore, clinic B uses different variations of sequences X and Y with different parameters (think TE and TR). The surgeons in clinic B are used to planning their surgery on those specific MRI sequences. So the exact situation from your experience happens: Even though a patient provides all the images from other clinics, he/she will have to get the imaging done all over again. Additionally, this introduces totally unnecessary costs to the health system and patient.
This non-standardization in imaging sequences is hair-raising at least. And guess what, in my experience it's not even consistent within a given clinic within a specific disease.
Thirdly, regarding your experience with DVDs, they actually do include the DICOM files (in some strangely named directory, usually). However, they also include a totally crappy portable image viewing program which usually runs as autostart.exe (malware may be easily introduced here, on a sidenote). This viewing program is often a down-specced, completely outdated version of the original imaging program at the clinic. Most doctors outside big clinics do not understand that it makes far more sense to extract the DICOMs into a good DICOM viewer (e.g. OsiriX) for further assessment. Only big clinics usually have some sort of IT department which extracts the DICOMs and loads them into their PACS.
Reference: I did my doctors thesis (Germany) in MRI research and am nearly graduated from med school. Happy to finally contribute to HN :)