He is very stressed and he is very stressful are 2 different things in English but they are both acceptable as answers. If you are stressed, you are suffering from stress but if you are stressful it means you cause stress to others
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Words using the root word “stress” can be confusing. In 1936, Selye, a medical researcher, described what is now called stress—the biological responses (effect) seen after exposure to a harmful agent (cause). Initially, the word “stress” was used for both the agent and the response. In 1975, Selye proposed calling the agent “stressor” and the response “stress.” Not all dictionaries and researchers have adopted Selye’s terms. This can explain the confusion about the use and meaning of “stressful.”
For more information, but not guanteed to make it any clearer), see: https://www.thefreedictionary.com/stress (noun definition 6) https://www.thefreedictionary.com/stressor https://www.thefreedictionary.com/stressful
According to a native Vietnamese speaker it means he is stressed himself. "Something made him nervous."
Typical mistake of vietnamese students learning English. They use 'stressful' when they mean 'stressed'