My mum died aged 45, nearly 20 years ago, and my dad - who was the same age - started dating within a year, and remarried about 9 months after that. A lot of relatives - especially on Mum's side of the family - found it hard, but I was pleased for him.
He was a rubbish single man, and they had "only" been married for 25 years. He neglected himself nutritionally, he stayed at work (self-employed) until 9 or 10 o'clock at night, and was quite simply a heart attack waiting to happen.
You will only ever have one mum, but we all know that multiple spouses are a possibility, and in this day age, a likelihood for a large %age of the population, but only a fraction of it due to the death of a much loved partner.
He's lonely, and spending time with you, although lovely, is not the same as spending time with a significant other. He's found someone who's making him happy, and is significantly older than my dad was (even now he is only in his mid 60's). I know it's hard to see your dad with someone else, but please be happy for him. He hasn't replaced your mum, he's making sure he's not sad and lonely (and maybe even viewing himself as a burden on you and any siblings you may have), as he gets further into his old age.
I feel for you, but please don't tell him. Speak to a close friend about it. Do you have one who's parent has also remarried after the death of their partner?