OK, presuming that the renting situation isn't particularly different to here.
I know a few Landlords of HMOs, they'll let someone view, they'll say they have kids, but the kids won't be staying. Next thing they know, kid is staying at weekends. There are either complaints, or they are aware that a child is there and it's murky with their insurances, or they just feel uncomfortable- and responsible. At that point, they give notice- but the persons joined the council list and it takes months and months, and months! To get them out. So this is probably why he isn't in a HMO.
Privately renting a flat? Yes an option, but he needs fairly good credit, and pass affordability. Then he also needs to seem like the best candidate from the LLs perspective.
LLs might prefer a couple,
LLs might prefer a "professional"
LLs might prefer someone without a child
LLs may just prefer someone who makes 10k more a year.
LLs may prefer someone who's turned up with a 6 month deposit- as has been referenced on MN a few times, people have had to do it because its so tough to get chosen.
In some areas of the country there are up to 50 people showing interest in any private rental that comes up.
There is talk of us having a housing crisis for a reason.