This guy was tracing patterns on his t-shirt with ultraviolet LEDs. The shirt was absorbing the light and wherever he traced, it would leave a glowing pattern that would eventually disappear. I asked him what he used and he said he used invisible ink. I ordered some invisible ink of my own and tried it on a shirt but the ink wasn't absorbing the light and leaving glowing trails. The only thing it did was glow if I shined the ultraviolet light over it but it would disappear as soon as I moved the light away or turned it off. Does anyone know what the guy used?

Most invisible inks are not Phosphorescent, but a few may be. The ink you used seems to be fluorescent, which is similar, but without the slow release of photons. Generally, glow in the dark items make use of a powdered phosphor either trapped in plastic or in paint applied to the item. Were there patterns on his shirt that could have been glow in the dark paint?

His t-shirt was white and it looked like there was a big yellow stain on the front of it but it didn't look like paint it was a faint yellow.

probably Zinc Sulphide based then.