Using a microwave to heat your baby's food can create "hot spots" that can burn his sensitive mouth. To be safe:
Test first Dish out a portion in a microwavable bowl and warm for only 5 to 30 seconds. Stir thoroughly and then taste it yourself to make sure it's not too hot.
Use the stove instead Heat water in a pan until it begins to steam but not boil, and put the jar in for 30 seconds. Then spoon out a portion, stir, and test.
Serve it at room temperature Only an open jar needs to be refrigerated. If it's a new jar, right off the shelf is fine. Your baby might even like squash better that way.