Bert, if your son had a friend die, and his family asked for mourners to wear (for example) a yellow tie - would he purposely wear a red one? Or if the family asked for donations in lieu of flowers to a Mental Health charity, would he donate (and publicise his donation) to a Cancer charity?
I'm thinking not.
People who have lost family, friends, or colleagues in Military Conflicts, and who many rely on the money/help/support that that RBL provides (because a succession of bastard Governments don't support them), would feel this was a needlessly political message at a Service to remember them.
It's not the place to do it.
The purchase of RBL poppies raises money to help those in Service, Veterans and the families of both. The purchase of white poppies raises money for the PPU. Wearing both together would be far less political and more respectful.
(And as for the HLS tweet - Whilst I don't believe that he has been in control of his own Twitter for many years now, the tone, content and language has changed enormously over the last few years, and often changes in between tweets, I also point out that he hasn't 'needed' the services that the RBL provides, so it's very easy for him to say "don't buy a RBL Poppy" - he obviously doesn't care about those who do, which doesn't make him a very nice person in my view)