Hi,
A bit of straining is fairly normal for anyone, including babies. even fully breastfed babies can have a bit of a grunt and squirm when they poo. Constipation refers to hard, dry, infrequent and pellety poos. is that what it is, or just a bit firmer than usual. your little one is young for food, so it's ok to wait a little longer to start solids. Make sure he has plenty of breast milk, and many people start with water around the same time as they start solids at 6mnths