You cant 'force' their hand as there is no law to say your 2 children cannot share a room anyway, however, you may find your council could help you if you apply to go on the waiting list.
My sister had a 14 year old daughter when she became pregnant again and she was opn the waiting list for a place. When she had her daughter her local council advised she could be allocated a 3 bedroomed place because of the age of her eldest and they did indeed offer her 3 bedrooms.
What have your council told you? TBH, your 27 year old son can quite easily get his own place
If this arrangement is to be long term, why not look for a mutual exchange? You will find this information at your local council or within other housing associations in your area.