Whether or not he can make a will?
1. Testamentary capacity (i.e. wits): it is fine if he drifts in and out of consciousness or if he is of poor mental health. The test is: (i) does he understand the nature of making a will and the fact that he is making a will? (ii) Does he comprehend roughly what his estate is? (e.g. can he remember how many houses he owns, whether he has any bank accounts, whether he has a large estate or a small one) and (iii) has he turned his mind to all the people who could reasonably lay claim to his estate? (e.g. family members, friends that he has maintained, children, de facto partner).
2. Get a medical certificate: where the will-maker is elderly, it is advised to get a doctor to certify as to his mental capacity to make a will.
How to make a will?
1. Hire a solicitor. Especially in this instance where he is elderly and he most likely has residency issues and possibly property overseas.
2. Have the lawyer go to your father in law and, in a private meeting, write down the instructions he gives the lawyer. You should not be giving instructions on his behalf, especially if you are a beneficiary. However, if you don't believe the will is likely to be contested, then you can be present and assist your father in law in giving the instructions.
3. The will needs to be witnessed by at least two witnesses. They should not be beneficiaries under the will.
If your father in law does not make a will, it is not fatal. Intestacy rules are quite fair and generally state that his estate goes to his spouse (or de facto partner which is counted as a "spouse" here) and children.