can he sit up? or does he have any standing aids?
if yes, how about an easel or blackboard(on a door-so low enough) and you could try and teach him to chalk/paint-more of a mark making exercise first but at least it will help him with hand /eye coordination.
even get a roll of cheap wall paper(use the reverse side) and some chunky crayons and sit and make marks.
how about one of the playmats(some like keyboards/stepping stones)that you walk/crawl/hit with hands to make a noise to encourage mobility(if this is appropriate)
water mat - when our dc was little we had a mat that you put water inside and she lay on it and could tap it/squidge it and it was a different sensory experience
my dc loved the fun song factory videos and we would sit together with our selection of instruments(sometimes) and join in or just sit together and sing- most probably not pc but encourages language development
swimming-find your local pool it may have a mums and tots session, if not just go yourself and have a relax with your ds in the water.It would be a good idea to talk to a superior member of staff as they may be able to offer good suggestions to you (and may have equipment that you can borrow to help support your ds)take your ds along the first time and just walk around the changing area/have a sit at the poolside and then the next time get in the water, build it up gradually try 5-10 mins the first time(and if that is ok) a little longer the next time but do build it up gradually as your child will tire just with the getting dressed and undressed too and you really don't want tears!
the park? indoor play areas-with friends?(some people hate them but all that nice soft equipment /balls is nice sensory)
Imaginative play can come later but if you want to start why not get a tea set and have a pretend picnic(but might frustrate ds if no drink in pretend cups) and talk ,name items as you pass them????(sorry it's a long time since my dc was 16 months)
have you seen a physiotherapist? been referred for portage or early intervention schemes? to get support as this will help you and give you confidence
my dc had problems with fine motor (and gross) and to encourage development try and get your ds to pass and reach for things across his mid line(iykwim)eg use right arm and put toy to the left(have someting in the other hand already or use for support)
oh and btw my child who doesn't have cp(have one that does too)didn't sit up/walk til late either so don't feel pressurised into doing things as'per the book', your ds is still a little baby and can still enjoy doing baby things for a wee while yet
sorry that is a bit long but i wish i had known about mumsnet when my dc were small as it is such a good resource of knowledge...
good luck!!