OP - Also for a very young child, I would suggest having an audiological evaluation and perhaps good vision screening just to rule out even slight deficits in those areas. If a child is not hearing at all frequencies this could impact speech development, too. In terms of what you can do as I noted in another post, keep your expectations wide open and give your DC as many opportunities to be with typical youngsters his/her age in natural settings. By this I mean going to library story times, using church or gym nurseries, going to playground and toddler pools in nice weather and as you can having play dates.
Our middle daughter who is a pediatric PT and has two very, very verbal children has had story time each night and in early years read the same books over and over. The only tv her kids had in earliest years were the Signing Time videos and especially for the five year old girl really did help bring on early communication skills. In terms of adaptive skill scores, I would put an emphasis on motor skills" pincer grip, play things which will encourage hand strengthening such as play doh, clay, directed play using say "color name"+ object to reinforce that abstract concept, in natural settings counting out objects, preschool settings will deal with the letter introduction and you could reenforce at home. Also just normal things of learning to feed oneself, to dress oneself as one goes along, to put away one's toys, to learn to share toys. Learning to follow one step and then two step directions is also another key.
I know waiting is very hard, but there is a reason why "official" clarification for testing for an intellectual or developmental disability does not happen until age 6. And if a child is fairly nonverbal at that time, it is key to get a place and a psychologist who has the background to test such a child. Here is one site with some milestones in terms of adaptive skills:
http://www.ldonline.org/article/6039/. Also, if DC has a special interest in say cars or animals then use that interest in terms of teaching basic skills including language, colors, numbers etc.