writing letter of support for H1-B visa applicant
If you can only talk about his grades from a few classes, have you considered telling the person that you might not be the best choice for a letter of support?

Even just a class can be enough to see a student's skill, for example if a project is a big component in the class or if the instructor closely examines sufficiently detailed assignments and exams. This is really the useful kind of component: "X is skilled at task Y, as evidenced by his work doing Z..." If you just say someone is skilled without much evidence, it won't be afforded much weight anyhow.

If there has been no actual work, you can at most comment on:


Their grasp of technical knowledge taught in the course
Their intellectual ability (how quickly they were able to learn)
Their discipline and professionalism (e.g. in showing up to class, studying and submitting assignments on time)
Their talent for that for this area of work based on their performance in the course


It sounds like not much, so as I said, perhaps you might not be the best person to write this.

If you really want to write the letter, I would advise asking fellow faculty if they have experience in such a situation and whether they can help you.