but I don't know how many GB of video card it should have.
By @Patrick34334016yxad
Look up the System Requirements page for each of the applications you listed. Note the one that has the highest Recommended specs for graphics hardware, and buy according to that. For example, the two most GPU-intensive applications on your list are Premiere Pro and Blender; looking at their system requirements, for Premiere 4–6GB of VRAM are recommended depending on the type of work, and Blender recommends 8GB, so you probably want to go with the higher number of 8GB VRAM to satisfy all applications.
But, also check the other applications in case one actually recommends more. Also, if your son will need to run multiple demanding applications at the same time, you might even kick up the specs a little more if multple applications need to share resources.