@gracie22,
I think he said 'don't tell me' for a reason.
Despite generally agreeing with what Coastalrat said - his views tends to be black and white...while humans come in all shades. There
are people who can handle and open relationship - but they are extremely rare.
There are a few more, who can handle an open relationship so long as they don't find out - and they definitely tend not to want to be told.
Personally, I think the odds are that he would be angry with you if you told him, and he'd every right to be...you agreed to something else.
If he finds out anyway, and is angry with you - you have every right to be angry with him...while understanding where his anger comes from.
In the end, I think 'do what you think is best, for the right reasons, and accept that there are consequences whatever you do' (some are good, some are bad, some depend solely on perspective).
Best of luck.