Thanks for bringing your question to JustAnswer.
When seizures are related to Fragile X, they usually begin in early childhood, and usually go away by the time the child becomes an adult, so it is unlikely that this seizure episode is related to Fragile X since it was his first one.
You may want to have him evaluated by a neurologist to see if anti-seizure medication will help him, if this hasn't already been prescribed.
I'd like to give you some links to specialized information about Fragile X in case you need additional information in the future on this diagnosis: I've chosen links that have at least some information on adults with Fragile X.
The National Fragile X Foundation: studies
A link to medications that are used for persons with Fragile X--notice that the first ones in the chart are for seizures.
National Institute for Health information on Fragile X
I hope this has helped! Click ACCEPT if this has answered your question, or if you need more info, let me know.