It depends what you mean by take it further?
If you mean find a way in house to make yourself confident that it won't happen again then no problem.
It is highly likely to be an issue if he does not understand why he shouldn't do it, it is highly unlikely that in this country in this day and age that leaving under 10's unsupervised at home would ever be considered acceptable parenting by the people that tend to have to consider that sort of thing regardless of how many people who don't have to decide these things disagree
And unfortunately he has now placed you in a position you don't wish to be in, yes he is an equal parent however he has done something you believe to be unacceptable he is appearing to lack understanding as to why so you cannot reasonably be sure he won't do it again therefore you could reasonably be expected to manage a known about risk.
you used the word 'safeguarding' so what,it is actually a word that is perfectly appropriate to use you do not have to have a special licence to ask the question, it's a mainstream term now.
I would ignore the people ridiculing you over your use of the word and it being in relation to a family member more children are exposed to safeguarding risks from their own family than any other situation and perhaps if more people where able to stand back from that family tie and did ask the question early enough perhaps people like me would see the words 'unable to protect' or 'lacks comprehension of issues raised' on various documents of the type that really matter to families.
Before anyone jumps all over this post and thinks it's intended to mean that you should go running off and reporting him to children's services or leaving him that is not what I am saying, Tho in this area a report of that nature would generate an actual visit as would it in the neighbouring areas. all I'm saying is you are not wrong for being concerned.
Sometimes loving caring decent parents do stupid things and sometimes the other parent needs to step in and either stop it or point out that it's stupid.
We like to think we have total and sole decision making power over our own kids and our own risk assessments are all that matters but in reality we don't and whilst we have services that get involved in these matters what they think matters far more becuase they can and do step in.