Sadly he has few rights.
Unless married or in a civil partnership, what happens to property and assets after the death of one partner will be determined by intestacy rules if they did not make a Will. These rules mean that the partner will be entitled to nothing from the deceased’s estate – their possessions and property will automatically pass to their next of kin (excepting situations where they have had the foresight to create a joint tenancy as detailed above). Depending on circumstances, this could be a less than desirable outcome – if no appropriate family can be found, the estate will even pass to the government over the surviving partner.
An unmarried partner to a deceased person will also not be able to apply to administer the estate of the deceased, due to not being a relative.
There is legal recourse for the partner, but he should still establish if there is a next of kin and speak with that person. They may be sympathetic if they knew the couple.