This is quite tricky. “Neither of” is usually followed by a term describing a plurality such as “twins”, “cousins”, “brothers” etc.
There is also the question of whether to stick to present tense (is) or change it to future tense (will be). It is generally a fine line but my rule of thumb is to stick to present tense for universal truths and future tense when describing a specific issue.
In this case, I would probably go with: Neither of the boys, John and William, will be attending the workshop.