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Hi. Frequent colds are common in children. 6 to 8 colds per year is usual. Children start speaking 2 to 3 words by 1 yr, 10 to 15 words by 1.5 yr and 2 word sentences by 2 yrs. So your child has a definite delay in speech. So needs evaluation including hearing test. Chewing thumb is not the reason for speech delay. Also Child needs to be taught rigorously at home how to speak by parents, grand parents etc. Can put him in play school as he will interact with other children and may learn fast. Speech delay is more common in small families where children has less opportunity to learn