Helen, it's good that you're observing your son's development closely and that you're concerned about his progress.
Children's development covers many domains—the physical or large motor development that you've described, small motor development (manipulating objects with their hands), emotional development and social relationships, language, and cognitive development (thinking). Children's development generally follows a pattern from simple to more complex—e.g., first sitting up, then crawling, pulling up on furniture, and walking. From observations of many children, experts have determined the age range at which most children reach these developmental milestones.
For example, typically developing babies usually sit up by 6-9 months, crawl by 9-12 months, and walk by 12-15 months of age. Since your child didn't sit up until 15 months of age, and is not crawling or walking at 18 months of age, he seems to have a delay in his physical development. That doesn't necessarily mean that your child is delayed in all areas of development. You described your baby as being â€œvery, very happyâ€?—that's a good sign for his emotional/social development.
Do you take your son to the doctor for regular check-ups and immunizations? Be sure to tell the doctor about your child's development and have the doctor examine him thoroughly. Ask to have your child evaluated by a developmental specialist to assess all of his areas of development and determine what interventions might be helpful to move his development forward. The sooner you do this, the better—studies have shown that children who get early intervention for developmental delays make the best progress.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.