It means that your Chart Ruler is Jupiter (provided the Ascendant is Sagittarius) so this is the most important planet for you. Also Pluto is very important as it's your Co-Ruler, those planets with also the Sun will be paramount in your life and you need to observe carefully the transits to them in your life.
If Jupiter is the 8th house or in Scorpio or in aspect to Pluto, this strengthen your Plutonian traits even more.
I don't think he has sag ascendant just because his pluto is in 1th house sag.
Everyone that's born between 1995-2008 has Pluto in Sagittarius. And Pluto is not like the Sun, only the Sun 1th house becomes the CO-Gov but not other planets. But of course 1th house is of the most importance [in a way] so it will have highly effect.
pluto in 1st house sagittarius what does this mean?
Well, look upon what the 1th house represents and means , this is the personality , one life , how he is viewed , his body, his ego, his 'himself' to the personality , Pluto being in the 1th house means your personality is very Plutonic, astrology is a [though] science but nobody should ever give up on it as it is the most divine thing of all.
To discover more about personality you got to look upon your planets in the 1th house , the actual 1th house in your natal chart [ meaning your ascendant ] and the Sun 'the others have things to say in one's personality'.
You can simply google as well as such ' Pluto in 1th house natal chart '.
Without complicating this to much and keeping it to your questions , in a way , it means that your personality is more Plutonic but with a sentence of Sagittarius[Jupiter] into it. This could be seen as a swapping from dark to light but all chart must be interrogate in full to fully decode yourself.