As you and your companion are not married, you are free to split the rental income in whatever proportions suit you so long as you have some interest in the property. Your name would need to be on the deeds and you would probably need to draw up a deed of trust to formalise the ownership percentages as my understanding is that the Land Registry record does not mention these, just the names of those who have an interest in the property including a mortgage lender. I'm not an expert on property law so if it were me, I'd consult a solicitor to find out what documentation needs to be drawn up and signed to split the property ownership.
You do need to consider the Capital Gains Tax implications of your companion gifting you an interest in his property. As this transaction will not be a "bargain at arm's length", the share of the property you take on will be deemed to have been sold to you at the open market value. In addition, as the property will have been your companion's main home and let, when it is sold he will be entitled to a combination of main residence relief and letting relief to reduce or wipe out his exposure to CGT. Assuming the property has never been your main home, you will not be entitled to either of those reliefs when the property is sold so your share of any gain will be taxable subject to the annual CGT exemption which is currently £10,900. Take a look at the HMRC helpsheet HS283 here
for information on the main residence and CGT.
Another consideration is stamp duty. If there is a mortgage on the property, if your share of that mortgage is more than £125,000, there will be a stamp duty charge. Take a look here
for more information on stamp duty. Your companion should also consult his lender about your plans.
I hope this helps but let me know if you have any further questions.