My dad had his cbc 3days ago and that was 105 but today it reduced to 100 thou/mm3 and his report says platelates are reduced and some giant one seen?

Low platelets...: ...and giant platelets can be due to several conditions. Some are serious -- not necessarily, but your dad should discuss these findings with his doctor ASAP. Depending on other symptoms, and why he had the CBC in the first place, he might need referral to a hematologist or other specialist. The change from 105K to 100K/mm3 is in the range of test variability, not necessarily worse than before.