Does your tenancy agreement have any restrictions on other parties occupying the property?
Check if there are any provisions for leaving the tenancy earlier than the initial 6 months, some more formal agreements do. Or speak to your Landlord and see if you can sort something out between you.
Your landlord should be interested as if the girlfriend and child move in then they are in "informal" occupation which could cause problems ifhe wants to sell, remortgage or otherwise deal with the property.
Even if you move out you are liable for your part of the rent under the tenancy agreement for the initial, I am guessing, 6 month period unless your landlord releases you early by varying the agreement or terminating the existing tenancy and granting a new tenancy to your housemate. Whatever happens ensure that it's all done properly and in writing and signed.
As there are people moving in then he may be fine to let this happen so long as the property is occupied and the rent is paid.
If you are joint tenants then you are both jointly and severally liable for the rent for this period so if any rent is unpaid then they can go after either of you. This could also apply to damage etc... they may want to recover money for.
I suppose it depends on what outcome you want, whether you are prepared to move out or want to stay and what problems it may cause with the guy you live with.
Bear in mind that if he got the huff and left then he is also liable for rent under a joint tenancy as detailed above unless he makes and agreement with the landlord. If he did a runner and didn't pay then the landlord can come after either of you so just be careful.