My seven month old grandson trembles sometimes for a second or two. Is this normal?

It really depends on. When it happens, what parts of his body & what other symptoms he displays. If while falling asleep, it's probably benign. If it stops his activity, is localized/one-sided, & he has low muscle tone (resistance to passive movement) with delays in fine & gross motor or cognitive & language development, it may be important. Seizures are usually rhythmical "jerks." video an episode for his pediatrician.