He can be harmed in his charged state. He can be harmed by large caliber guns, explosives, missiles, etc. He can be harmed by people like Mammoth. He is completely vulnerable to magic and psychic users.

I wasn't able to find much on RedStar but I did come up with this.

If he receives a large influx of power it would be quite painful and he would be rendered unconscious until his body had a chance to process it. If he was fully charged and continued to take in energy, he would have to convert it and quickly expel it or risk falling in to a coma and “detonating” after about two weeks if the energy is not expelled.

I also was toying with some other ideas. I'm trying to make sure he's not overpowered while still making him useful to the Titans but this process is helping me get out the kinks.