hi
first of all you dont have to worry "to" much about the dalayed walking. some childs need a bit longer. if you have the feeling that the muscle tone is decreased you should definately consult a physio. physiotherapy is able to increase the muscle tone and also to improve motor skills etc. a good physio also might find out other conditions or reasons for the dalying.
for example has your kid ever had problems with the cervical spine, i.e. kiss syndrome or torticollis myogenens. sometimes pain can cause problems with the development. also hip joint should be clear of any malformations.
but most important is to find a good physio who can sort out the problem. from my experience its better than most physicians!! sorry doc´s!!
good luck and all the best for your kid!!