Were both the DLA and PIP payments paid into a bank account, and if so were they for different amounts?
Checking the statements should give you the info: also the code for who paid the money in will be different. eg: we get payments for AA and State Pension and although they both come from DWP the code for the payments is different.
Can’t the DLA department actually tell you his NI number? If your email them your request will be forwarded to the relevant information owner within the Department who will respond to you by the next working day.
If he is receiving any benefit, the NI number is usually printed somewhere on the correspondence. It’s the same number throughout life.
I’m a really “sad” person! I’ve known my husband’s NI number for at least 30 years off by heart, and also my disabled son’s. More recently, I can remember mine. Writing it down a few times seems to burn it into my brain hard drive. Introduce me to someone at a party, I’ve forgotten their name in a second!!!