I'm big on meaning, so I would personally go for a middle name that would direct or balance an ambiguous/possibly negative first name meaning like Hamish - "He who supplants (or usurps)". Although, it most probably would be fitting in some ways as I'm sure your two oldest may feel like he will be usurping the attention of mum and dad when he first comes along...

i.e.'s
Jason - healer (I know it has an 's' but the meaning is lovely in conjunction with Hamish)
Alexander - man's defender
Brendan - prince
Calum - dove

So for instance, the name Hamish Jason could be said to mean he provides healing by supplanting any pain or negative history. You get the idea...