It's not a simple problem that can be consulted online
You definitely need to consult a paediatrician and if possible a paediatric neurologist
We need to do few tests to rule out muscle problems
This is called delayed milestones
We also need to do few blood tests
And if the initial blood tests and scans reveal any issue then we also might need a genetic tests
Next Steps
consult a paediatric Neurologist
Health Tips
Start physiotherapy..that will give him some improvement
Do it ASAP