It sounds to me that this is probably something that SHOULD concern you HOWEVER you will probably get no REAL answers about it until he turns about a year old. First Steps is a WONDERFUL IDEA and he will probably get labeled YCDD (young child with developmental delays) and that does not really mean anything, when he turns 3 you can put him in your local public school districts program for children with small or large delays called Early childhood, but right now, I would just work with him AS MUCH as possible, my son did not sit up until he was 10 months and did not walk until he was 19 months and did not want to eat textured foods until he was 2. (I would pay close attention to how he responds to different textures) My son is now 5 years old and spends 360 minutes a week in different therapies although he is in a regular kindergarten setting, after they turn 5 and they are still considered YCDD and have an IEP (papers they get that establish their goals and what they have achieved) then my son got the label Mentally Retarded, WHICH I DO NOT SEE!!!!! But when they turn 5 if it is still a concern that is the label they get, do not let me scare you, just try and take as many steps as possible to attempt to get your son to do the things you are wanting him to do, lay him on his tummy and find something he really likes, and hold it in front of his face, make him reach for it and then take it over his head and he will more than likely roll over to try and get it, then do it the other way, do this several times a day until hopefully he just gets it on his own, as for sitting up, put something in his hands that he does not usually lay down with for example if you give him a bottle he is more than likely going to lean back because that is how he gets the milk, but if you give him something that catches his attention like a ball that lights up and plays music and is big enough to go between his legs he will probably try to sit up without even realizing it. He is not just going to go from laying to sitting, you are just going to have to sit him up and give him reason to want to stay in that posiition. Also praise him with lots of hugs and kisses and things he may like....as he gets a little bigger cheerios will be in order, as most kids really take well to them lol!!!
Good luck and if you need anything dont hesititate to send me an personal message! I have a myspace as well as yahoo messenger! Talk to you soon!