Thomas' weaknesses may be a bit obvious, but here's the list:
-First off, he is slowly going insane. His power drives his own mind crazy with fear and has caused him to develop slight dementia and random spurts of insomnia.
-Due to the dementia, he is more prone to his own power, causing him to see random visions at random times. If he can catch it quick enough, he is able to just rub away the visions, but if not, he gets to where he is living a different version of the same moment, with nightmares coming to life withing his own mind.
-Due to the insomnia, it is harder for him to differentiate the reality from the fear ability when it comes upon himself. This makes him even more prone to not catch the fear soon enough.
-Also, due to the dementia, he tends to go a little overboard with his power. If all he needs is the vision of a death of another to be implanted in someones mind, he will cause a river or even ocean of blood to come spilling forth. If he just needs them to think they are about to get burned, he may up it to have already lit their arm on fire inside their mind. Etcetera etcetera.