The first thing that strikes me is that your dm has no concern for you or your ds feelings. She doesn't care he will miss you and you him. is far too young to be allowed to fly, long haul with Gran? and thence be hauled round a strange country by a bunch of strange people - yes they well be relatives but they are still strangers who bizarrely will behave in your sons eyes by grabbing him, kissing him, all in a strange language. Where will he sleep? Alone he will cry, or will he be sharing a bed with granny? Or strange children? You won't know, will you? He will ne potty training about then to, but he may regress. Or will she do t for you, trumping you forever?
It is not her Job to show him off, and why should she? You have Skype surely, for all these relatives eager to see him- why all of a sudden? Do yo have close contact and know all these people your ds may see?
Passport- his country of residence is uk, he is british. Why on earth would ds need a Colombian passport? What passport does your dp have, and mil? And it's not 'nice, or come in useful' - you wouldn't move freely round Europe with him, and he won't be able to either when older , loads of reasons stretching into your sons adult life. Having a uk passport in incredibly beneficial.
My dc dont spend time alone with dgp, as they live in anther country and the dc don't want to, even though they are lovely and we stay often for prolonged periods since birth of dc's . And we all respect that, even though it can be annoying. Hs they feel is very real and it would be cruel to discredit it just for adult gain.
Hide passports and birth certs... Yeah, I would do that.