Sorry for making this an answer – if I had enough reputation to comment, I would rather write a comment on @Arsak's fine transcription, as (s)he did most of the work. Here's just building upon @Arsak's answer.
Deletions and insertions are < marked > as shown.
I have also added some remarks (below).
Lieber Bruder
Jetz komme ich an der Reihe, ich bin
noch immer Gesund und Munter,
mir gehts gut und wohl. Ich habe die
Höhe von 5 Fuß erreicht, welches mir
sehr freut, auch bin Ich mit vielen
Richten[?] umgeben, und ungefähr
[95|75] ℔ [= Pfund] schwer. < Im > Nächsten Brief werde
ich mein Bild hinschicken, und in diesem
Brief werdet Ihr wirst Du Fritz Bild mit
Freuden im Empfang nehmen. Wir
haben jetzt einen neuen Lehrer wieder
namens Doose, dieser kann aber nicht
so viel als Herr Münster. Ich komme[?]
um[?] 2 Jahre aus der Schule. Auch habe ich viele
Fische gefangen, denn es[?] wir[?] []
Br gewölbte Brüke < gebadet >. Für dießmal
will ich schließen, weil der Brief sonst
zuviel Geld kostet. Dein Bruder
Heinrich
The document was probably a collective letter from different family members, and seeing as Heinrich's letter is written on the verso page of a sheet, it might have been the last in the lot and at the very end of a Bogen. Hence his worry that writing an additional page would require another sheet and cost more to send.
The symbol on l. 8 ought to be a (pretty standard) abbreviation for Pfund.
A Pfund in the German Reich was defined as weighing exactly 0.5 kg.
So the boy is five Fuß tall and weighs either 47.5 or 37.5 kg (95 or 75 Pfund).
The first word on l. 7 looks like "Richten" to me; I don't find it completely satisfactory, but here's a rationalization.
There is a documented dialectal (esp. niederdeutsch) usage of das Richt or die Richte as equivalent to "Speise", "Gericht", "Mahlzeit" (DWB).
If the superimposed heading is correct ("Trittau den 26 Juli 1872"), the letter could have been written in a town in Schleswig-Holstein near Hamburg.
Thus, after stating his height, Heinrich would be saying "auch bin Ich mit vielen Richten umgeben", meaning he has plenty to eat; then he states his current weight.
Even then, a sentence such as "ich bin mit vielen Speisen umgeben" sounds clumsy to me; I can only speculate about it being a regional idiom, or standard back then, or peculiar to the writer.
(Other deviations from present-day grammar: "Jetz komme ich an der Reihe" and "welches mir sehr freut".)
I could not decipher certain words in the part about fishing and swimming near a "gewölbte Brüke" (l. 15-17); also, I am unsure about "komme" at the end of l. 14, as I don't see a typical, closed "o".