I have been with my girlfriend for two years and when my grandma died last year, she left me a small inheritance (certainly compared to his!!) My family told me specifically not to tell anyone else about it including my girlfriend - not due to any particular suspicion of her but just that it's none of anyone's business what money someone comes into or earns at their job. That is fair enough and in such a situation you would have to do what the family want. You cannot be too careful with your money. The era of the joint account and joint mortgage is long past.
Now, of course, I have more or less let my girlfriend know about it, but haven't told her the exact amount and haven't made a big song and dance about it.
Certainly after only a few weeks he is still sorting out what to do with the money himself and adjusting to vastly altered perceptions of his own future. He shouldn't be browbeaten for not telling you - it's not like he could have intended to keep it from you for life.
Also I would hope you wouldn't stay with him for the money or anything, or attempt to get it spent on yourself. If that's the case then how much he has shouldn't affect you in the slightest!
It's just being grown-up and realistic and seeing your relationship in the wider/long-term context. At the risk of offending you I do think girls set too much store by their relationship and lose their sense of perspective.